diff --git a/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt b/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt
index fec618093d2..7dfeb4f72bd 100644
--- a/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt
+++ b/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt
@@ -579,7 +579,7 @@ internal class IterableLoopHeader(
         loopVariable: IrVariable?,
         loopVariableComponents: Map<Int, IrVariable>,
         builder: DeclarationIrBuilder
-    ) =
+    ): List<IrStatement> =
         with(builder) {
             // loopVariable = iteratorVar.next()
             val iteratorClass = headerInfo.iteratorVariable.type.getClass()!!
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ES6ConstructorLowering.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ES6ConstructorLowering.kt
index ff2028ea3bd..cdcfa1704a2 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ES6ConstructorLowering.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ES6ConstructorLowering.kt
@@ -386,7 +386,7 @@ private fun replaceCallToDefaultPrimary(context: JsIrBackendContext, constructor
     val thisSymbol = (((constructor.body as IrBlockBody).statements
         .find { it is IrVariable && it.origin === ES6_THIS_VARIABLE_ORIGIN }) as IrVariable?)?.symbol ?: return
 
-    (constructor.body as IrBlockBody).statements.transform {
+    (constructor.body as IrBlockBody).statements.transformInPlace {
         if (it is IrDelegatingConstructorCall) {
             val superCtor = it.symbol.owner
             val initFunc = context.mapping.constructorToInitFunction[superCtor]!!
@@ -501,7 +501,7 @@ private class LowerCtorHelper(
             origin = ES6_THIS_VARIABLE_ORIGIN
         }
 
-        (constructor.body as IrBlockBody).statements.transform {
+        (constructor.body as IrBlockBody).statements.transformInPlace {
             if (it === superCall) newThis
             else it
         }
@@ -561,7 +561,7 @@ private fun changeIrConstructorToIrFunction(context: JsIrBackendContext, contain
         }, null)
     }
 
-    container.parentAsClass.declarations.transform {
+    container.parentAsClass.declarations.transformInPlace {
         if (it === container) newConstructor else it
     }
 
diff --git a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/AnonymousObjectSuperConstructorLowering.kt b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/AnonymousObjectSuperConstructorLowering.kt
index 9ae4ebbcede..5c298df7a12 100644
--- a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/AnonymousObjectSuperConstructorLowering.kt
+++ b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/AnonymousObjectSuperConstructorLowering.kt
@@ -21,7 +21,7 @@ import org.jetbrains.kotlin.ir.declarations.IrVariable
 import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.IrConstructorCallImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrGetValueImpl
-import org.jetbrains.kotlin.ir.util.transform
+import org.jetbrains.kotlin.ir.util.transformInPlace
 
 internal val anonymousObjectSuperConstructorPhase = makeIrFilePhase(
     ::AnonymousObjectSuperConstructorLowering,
@@ -91,7 +91,7 @@ private class AnonymousObjectSuperConstructorLowering(val context: JvmBackendCon
             }
         }
 
-        objectConstructorBody.statements.transform {
+        objectConstructorBody.statements.transformInPlace {
             when {
                 it is IrDelegatingConstructorCall -> it.transform(listOf())
                 it is IrBlock && it.origin == IrStatementOrigin.ARGUMENTS_REORDERING_FOR_CALL && it.statements.last() is IrDelegatingConstructorCall ->
diff --git a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InheritedDefaultMethodsOnClassesLowering.kt b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InheritedDefaultMethodsOnClassesLowering.kt
index 728a29acdb7..a184a7e2ede 100644
--- a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InheritedDefaultMethodsOnClassesLowering.kt
+++ b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InheritedDefaultMethodsOnClassesLowering.kt
@@ -61,7 +61,7 @@ private class InheritedDefaultMethodsOnClassesLowering(val context: JvmBackendCo
     }
 
     private fun generateInterfaceMethods(irClass: IrClass) {
-        irClass.declarations.transform { declaration ->
+        irClass.declarations.transformInPlace { declaration ->
             (declaration as? IrSimpleFunction)?.findInterfaceImplementation(context.state.jvmDefaultMode)?.let { implementation ->
                 generateDelegationToDefaultImpl(implementation, declaration)
             } ?: declaration
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrClass.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrClass.kt
index 0b52383502a..356e8674162 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrClass.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrClass.kt
@@ -23,7 +23,7 @@ import org.jetbrains.kotlin.descriptors.SourceElement
 import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
 import org.jetbrains.kotlin.ir.types.IrType
-import org.jetbrains.kotlin.ir.util.transform
+import org.jetbrains.kotlin.ir.util.transformInPlace
 import org.jetbrains.kotlin.ir.util.transformIfNeeded
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
@@ -63,7 +63,7 @@ abstract class IrClass :
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
         thisReceiver = thisReceiver?.transform(transformer, data)
         typeParameters = typeParameters.transformIfNeeded(transformer, data)
-        declarations.transform { it.transform(transformer, data) }
+        declarations.transformInPlace(transformer, data)
     }
 }
 
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt
index 0667e64bec7..e161cf36d10 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt
@@ -20,6 +20,7 @@ import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibility
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.IrElementBase
 import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
@@ -45,7 +46,7 @@ interface IrDeclaration : IrStatement, IrMutableAnnotationContainer {
     val factory: IrFactory
 }
 
-abstract class IrDeclarationBase : IrDeclaration {
+abstract class IrDeclarationBase : IrElementBase(), IrDeclaration {
     override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrStatement =
         accept(transformer, data) as IrStatement
 }
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrScriptImpl.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrScriptImpl.kt
index dbd35327cbb..50a7d8db8a9 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrScriptImpl.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrScriptImpl.kt
@@ -12,7 +12,7 @@ import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.IrConstructorCall
 import org.jetbrains.kotlin.ir.symbols.IrScriptSymbol
-import org.jetbrains.kotlin.ir.util.transform
+import org.jetbrains.kotlin.ir.util.transformInPlace
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 import org.jetbrains.kotlin.name.Name
@@ -65,8 +65,8 @@ class IrScriptImpl(
     }
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
-        declarations.transform { it.transform(transformer, data) }
-        statements.transform { it.transform(transformer, data) }
+        declarations.transformInPlace(transformer, data)
+        statements.transformInPlace(transformer, data)
         thisReceiver = thisReceiver.transform(transformer, data)
     }
 }
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBlock.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBlock.kt
index d2392847bf3..0eba2a78492 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBlock.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBlock.kt
@@ -23,7 +23,7 @@ import org.jetbrains.kotlin.ir.symbols.IrFileSymbol
 import org.jetbrains.kotlin.ir.symbols.IrFunctionSymbol
 import org.jetbrains.kotlin.ir.symbols.IrReturnableBlockSymbol
 import org.jetbrains.kotlin.ir.util.file
-import org.jetbrains.kotlin.ir.util.transform
+import org.jetbrains.kotlin.ir.util.transformInPlace
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 
@@ -38,7 +38,7 @@ abstract class IrContainerExpression : IrExpression(), IrStatementContainer {
     }
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
-        statements.transform { it.transform(transformer, data) }
+        statements.transformInPlace(transformer, data)
     }
 }
 
@@ -58,5 +58,6 @@ abstract class IrReturnableBlock : IrBlock(), IrSymbolOwner, IrReturnTarget {
     abstract val inlineFunctionSymbol: IrFunctionSymbol?
 }
 
+@Suppress("unused") // Used in kotlin-native
 val IrReturnableBlock.sourceFileSymbol: IrFileSymbol?
     get() = inlineFunctionSymbol?.owner?.file?.symbol
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/transform.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/transform.kt
index 8f9aca58b26..77496f9351e 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/transform.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/transform.kt
@@ -17,13 +17,22 @@
 package org.jetbrains.kotlin.ir.util
 
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.IrElementBase
 import org.jetbrains.kotlin.ir.declarations.IrDeclaration
 import org.jetbrains.kotlin.ir.declarations.IrDeclarationContainer
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 
-inline fun <reified T : IrElement> MutableList<T>.transform(transformation: (T) -> IrElement) {
-    forEachIndexed { i, item ->
-        set(i, transformation(item) as T)
+inline fun <reified T : IrElement> MutableList<T>.transformInPlace(transformation: (T) -> IrElement) {
+    for (i in 0 until size) {
+        set(i, transformation(get(i)) as T)
+    }
+}
+
+fun <T : IrElement, D> MutableList<T>.transformInPlace(transformer: IrElementTransformer<D>, data: D) {
+    for (i in 0 until size) {
+        // Cast to IrElementBase to avoid casting to interface and invokeinterface, both of which are slow.
+        @Suppress("UNCHECKED_CAST")
+        set(i, (get(i) as IrElementBase).transform(transformer, data) as T)
     }
 }