diff --git a/compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/AbstractModularizedTest.kt b/compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/AbstractModularizedTest.kt
index 95eacbff39b..3c12f6d35c3 100644
--- a/compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/AbstractModularizedTest.kt
+++ b/compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/AbstractModularizedTest.kt
@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir
 
 import org.jetbrains.kotlin.fir.scopes.ProcessorAction
 import org.jetbrains.kotlin.test.testFramework.KtUsefulTestCase
+import org.jetbrains.kotlin.types.AbstractTypeChecker
 import org.w3c.dom.Node
 import org.w3c.dom.NodeList
 import java.io.File
@@ -32,6 +33,16 @@ private fun NodeList.toList(): List<Node> {
 private val Node.childNodesList get() = childNodes.toList()
 
 abstract class AbstractModularizedTest : KtUsefulTestCase() {
+    override fun setUp() {
+        super.setUp()
+        AbstractTypeChecker.RUN_SLOW_ASSERTIONS = false
+    }
+
+    override fun tearDown() {
+        super.tearDown()
+        AbstractTypeChecker.RUN_SLOW_ASSERTIONS = true
+    }
+
     private fun loadModule(file: File): ModuleData {
 
         val factory = DocumentBuilderFactory.newInstance()
@@ -92,4 +103,4 @@ abstract class AbstractModularizedTest : KtUsefulTestCase() {
 
         afterPass()
     }
-}
\ No newline at end of file
+}
diff --git a/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt b/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt
index 0b1d564258b..6159ddbf36b 100644
--- a/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt
+++ b/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt
@@ -253,6 +253,7 @@ abstract class AbstractTypeCheckerContextForConstraintSystem : AbstractTypeCheck
         AbstractTypeChecker.isSubtypeOf(this as AbstractTypeCheckerContext, subType, superType)
 
     private fun assertInputTypes(subType: KotlinTypeMarker, superType: KotlinTypeMarker) {
+        if (!AbstractTypeChecker.RUN_SLOW_ASSERTIONS) return
         fun correctSubType(subType: SimpleTypeMarker) =
             subType.isSingleClassifierType() || subType.typeConstructor().isIntersection() || isMyTypeVariable(subType) || subType.isError() || subType.isIntegerLiteralType()
 
@@ -278,4 +279,4 @@ abstract class AbstractTypeCheckerContextForConstraintSystem : AbstractTypeCheck
         is FlexibleTypeMarker -> f(lowerBound()) || f(upperBound())
         else -> error("sealed")
     }
-}
\ No newline at end of file
+}
diff --git a/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/NewTypeSubstitutor.kt b/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/NewTypeSubstitutor.kt
index 2a378dc6314..6773eddf166 100644
--- a/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/NewTypeSubstitutor.kt
+++ b/compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/NewTypeSubstitutor.kt
@@ -88,13 +88,15 @@ interface NewTypeSubstitutor: TypeSubstitutorMarker {
                         "original lower type: '${capturedType.lowerType}"
             )
 
-            typeConstructor.supertypes.forEach { supertype ->
-                substitute(supertype, keepAnnotation, runCapturedChecks = false)?.let {
-                    throw IllegalStateException(
-                        "Illegal type substitutor: $this, " +
-                                "because for captured type '$type' supertype approximation should be null, but it is: '$supertype'," +
-                                "original supertype: '$supertype'"
-                    )
+            if (AbstractTypeChecker.RUN_SLOW_ASSERTIONS) {
+                typeConstructor.supertypes.forEach { supertype ->
+                    substitute(supertype, keepAnnotation, runCapturedChecks = false)?.let {
+                        throw IllegalStateException(
+                            "Illegal type substitutor: $this, " +
+                                    "because for captured type '$type' supertype approximation should be null, but it is: '$supertype'," +
+                                    "original supertype: '$supertype'"
+                        )
+                    }
                 }
             }
 
@@ -178,4 +180,4 @@ class FreshVariableNewTypeSubstitutor(val freshVariables: List<TypeVariableFromC
 fun UnwrappedType.substituteTypeVariable(typeVariable: NewTypeVariable, value: UnwrappedType): UnwrappedType {
     val substitutor = NewTypeSubstitutorByConstructorMap(mapOf(typeVariable.freshTypeConstructor to value))
     return substitutor.safeSubstitute(this)
-}
\ No newline at end of file
+}
diff --git a/compiler/tests-common/tests/org/jetbrains/kotlin/test/testFramework/KtUsefulTestCase.java b/compiler/tests-common/tests/org/jetbrains/kotlin/test/testFramework/KtUsefulTestCase.java
index c2f3345264d..f387166ed58 100644
--- a/compiler/tests-common/tests/org/jetbrains/kotlin/test/testFramework/KtUsefulTestCase.java
+++ b/compiler/tests-common/tests/org/jetbrains/kotlin/test/testFramework/KtUsefulTestCase.java
@@ -16,8 +16,6 @@
 
 package org.jetbrains.kotlin.test.testFramework;
 
-import com.intellij.codeInsight.CodeInsightSettings;
-import com.intellij.concurrency.IdeaForkJoinWorkerThreadFactory;
 import com.intellij.diagnostic.PerformanceWatcher;
 import com.intellij.openapi.Disposable;
 import com.intellij.openapi.application.Application;
@@ -51,6 +49,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.kotlin.test.IdeaSystemPropertiesForParallelRunConfigurator;
 import org.jetbrains.kotlin.testFramework.MockComponentManagerCreationTracer;
+import org.jetbrains.kotlin.types.AbstractTypeChecker;
 import org.jetbrains.kotlin.types.FlexibleTypeImpl;
 import org.jetbrains.kotlin.utils.ExceptionUtilsKt;
 import org.junit.Assert;
@@ -100,6 +99,7 @@ public abstract class KtUsefulTestCase extends TestCase {
         System.setProperty("apple.awt.UIElement", "true");
 
         FlexibleTypeImpl.RUN_SLOW_ASSERTIONS = true;
+        AbstractTypeChecker.RUN_SLOW_ASSERTIONS = true;
     }
 
     protected boolean shouldContainTempFiles() {
@@ -981,4 +981,4 @@ public abstract class KtUsefulTestCase extends TestCase {
             return KtUsefulTestCase.this.getClass() + (StringUtil.isEmpty(testName) ? "" : ".test" + testName);
         }
     };
-}
\ No newline at end of file
+}
diff --git a/compiler/tests/org/jetbrains/kotlin/jvm/compiler/FlexibleTypeAssertionsEnabledTest.kt b/compiler/tests/org/jetbrains/kotlin/jvm/compiler/SlowTypeAssertionsEnabledTest.kt
similarity index 54%
rename from compiler/tests/org/jetbrains/kotlin/jvm/compiler/FlexibleTypeAssertionsEnabledTest.kt
rename to compiler/tests/org/jetbrains/kotlin/jvm/compiler/SlowTypeAssertionsEnabledTest.kt
index 307bce216f5..a65b9ba326e 100644
--- a/compiler/tests/org/jetbrains/kotlin/jvm/compiler/FlexibleTypeAssertionsEnabledTest.kt
+++ b/compiler/tests/org/jetbrains/kotlin/jvm/compiler/SlowTypeAssertionsEnabledTest.kt
@@ -18,11 +18,15 @@ package org.jetbrains.kotlin.jvm.compiler
 
 import org.jetbrains.kotlin.builtins.DefaultBuiltIns
 import org.jetbrains.kotlin.test.KotlinTestWithEnvironmentManagement
+import org.jetbrains.kotlin.types.AbstractNullabilityChecker
 import org.jetbrains.kotlin.types.KotlinTypeFactory
+import org.jetbrains.kotlin.types.SimpleType
+import org.jetbrains.kotlin.types.TypeIntersector
+import org.jetbrains.kotlin.types.checker.ClassicTypeCheckerContext
 
-class FlexibleTypeAssertionsEnabledTest : KotlinTestWithEnvironmentManagement() {
+class SlowTypeAssertionsEnabledTest : KotlinTestWithEnvironmentManagement() {
 
-    fun testAssertionsAreOn() {
+    fun testAssertionsForFlexibleTypesAreOn() {
         val builtIns = DefaultBuiltIns.Instance
 
         try {
@@ -34,4 +38,21 @@ class FlexibleTypeAssertionsEnabledTest : KotlinTestWithEnvironmentManagement()
 
         fail("Assertion error expected")
     }
+
+    fun testAssertionsForTypeCheckerAreOn() {
+        val builtIns = DefaultBuiltIns.Instance
+
+        try {
+            val superType = TypeIntersector.intersectTypes(listOf(builtIns.charSequence.defaultType, builtIns.comparable.defaultType))
+            AbstractNullabilityChecker.isPossibleSubtype(
+                ClassicTypeCheckerContext(errorTypeEqualsToAnything = true), builtIns.annotationType,
+                superType as SimpleType
+            )
+        } catch (e: AssertionError) {
+            assertEquals("Not singleClassifierType superType: {CharSequence & Comparable<T>}", e.message)
+            return
+        }
+
+        fail("Assertion error expected")
+    }
 }
diff --git a/core/type-system/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt b/core/type-system/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt
index fa8a711a2a3..cb31e57ea16 100644
--- a/core/type-system/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt
+++ b/core/type-system/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt
@@ -144,6 +144,8 @@ abstract class AbstractTypeCheckerContext : TypeSystemContext {
 }
 
 object AbstractTypeChecker {
+    @JvmField
+    var RUN_SLOW_ASSERTIONS = false
 
     fun isSubtypeOf(context: TypeCheckerProviderContext, subType: KotlinTypeMarker, superType: KotlinTypeMarker): Boolean {
         return AbstractTypeChecker.isSubtypeOf(context.newBaseTypeCheckerContext(true), subType, superType)
@@ -223,11 +225,13 @@ object AbstractTypeChecker {
         }
 
     private fun AbstractTypeCheckerContext.isSubtypeOfForSingleClassifierType(subType: SimpleTypeMarker, superType: SimpleTypeMarker): Boolean {
-        assert(subType.isSingleClassifierType() || subType.typeConstructor().isIntersection() || subType.isAllowedTypeVariable) {
-            "Not singleClassifierType and not intersection subType: $subType"
-        }
-        assert(superType.isSingleClassifierType() || superType.isAllowedTypeVariable) {
-            "Not singleClassifierType superType: $superType"
+        if (AbstractTypeChecker.RUN_SLOW_ASSERTIONS) {
+            assert(subType.isSingleClassifierType() || subType.typeConstructor().isIntersection() || subType.isAllowedTypeVariable) {
+                "Not singleClassifierType and not intersection subType: $subType"
+            }
+            assert(superType.isSingleClassifierType() || superType.isAllowedTypeVariable) {
+                "Not singleClassifierType superType: $superType"
+            }
         }
 
         if (!AbstractNullabilityChecker.isPossibleSubtype(this, subType, superType)) return false
@@ -469,12 +473,14 @@ object AbstractNullabilityChecker {
         }
 
     private fun AbstractTypeCheckerContext.runIsPossibleSubtype(subType: SimpleTypeMarker, superType: SimpleTypeMarker): Boolean {
-        // it makes for case String? & Any <: String
-        assert(subType.isSingleClassifierType() || subType.typeConstructor().isIntersection() || subType.isAllowedTypeVariable) {
-            "Not singleClassifierType and not intersection subType: $subType"
-        }
-        assert(superType.isSingleClassifierType() || superType.isAllowedTypeVariable) {
-            "Not singleClassifierType superType: $superType"
+        if (AbstractTypeChecker.RUN_SLOW_ASSERTIONS) {
+            // it makes for case String? & Any <: String
+            assert(subType.isSingleClassifierType() || subType.typeConstructor().isIntersection() || subType.isAllowedTypeVariable) {
+                "Not singleClassifierType and not intersection subType: $subType"
+            }
+            assert(superType.isSingleClassifierType() || superType.isAllowedTypeVariable) {
+                "Not singleClassifierType superType: $superType"
+            }
         }
 
         // superType is actually nullable
@@ -525,4 +531,4 @@ object AbstractNullabilityChecker {
         }) {
             if (it.isMarkedNullable()) SupertypesPolicy.None else SupertypesPolicy.LowerIfFlexible
         }
-}
\ No newline at end of file
+}