diff --git a/Compilation/EmittedRuntime.cs b/Compilation/EmittedRuntime.cs
index 9276bb30..8d5b81ce 100644
--- a/Compilation/EmittedRuntime.cs
+++ b/Compilation/EmittedRuntime.cs
@@ -1586,6 +1586,10 @@ public class EmittedRuntime
     public MethodBuilder TypedArrayGetBuffer { get; set; } = null!;
     public MethodBuilder TypedArrayElementGet { get; set; } = null!;
     public MethodBuilder TypedArrayElementSet { get; set; } = null!;
+    // Unboxed Float64Array element accessors (#878) — direct double get/set on the
+    // concrete $Float64Array, bypassing the boxed Get/Set + GetIndex dispatch.
+    public MethodBuilder Float64ArrayGetUnboxed { get; set; } = null!;
+    public MethodBuilder Float64ArraySetUnboxed { get; set; } = null!;
 
     // Concrete TypedArray types (pure-IL for standalone DLLs)
     public TypeBuilder Int8ArrayType { get; set; } = null!;
diff --git a/Compilation/ILEmitter.Properties.cs b/Compilation/ILEmitter.Properties.cs
index 54920777..31a7ec9d 100644
--- a/Compilation/ILEmitter.Properties.cs
+++ b/Compilation/ILEmitter.Properties.cs
@@ -858,6 +858,25 @@ protected override void EmitGetIndex(Expr.GetIndex gi)
             return;
         }
 
+        // Float64Array fast path (#878): when the receiver is a variable statically typed
+        // Float64Array, read the element UNBOXED via $Float64Array.GetUnboxed → native double
+        // on the stack. Eliminates the Runtime.GetIndex dispatch, the GetTypedArrayElement
+        // isinst/castclass, the virtual Get, and the per-element box. Out-of-range access
+        // faults via BitConverter exactly as the boxed path does today (OOB semantics
+        // unchanged). Receiver is a side-effect-free variable, so it is loaded once.
+        if (!gi.Optional && gi.Object is Expr.Variable
+            && _ctx.TypeMap?.Get(gi.Object) is TypeInfo.TypedArray { ElementType: "Float64" })
+        {
+            EmitExpression(gi.Object);
+            EnsureBoxed();
+            IL.Emit(OpCodes.Castclass, _ctx.Runtime!.Float64ArrayType);
+            EmitExpressionAsDouble(gi.Index);
+            IL.Emit(OpCodes.Conv_I4);
+            IL.Emit(OpCodes.Callvirt, _ctx.Runtime!.Float64ArrayGetUnboxed);
+            SetStackType(StackType.Double);
+            return;
+        }
+
         // Descriptor-driven fast path: when receiver is statically known to be an array,
         // emit direct List<T> access — skips runtime type dispatch,
         // index boxing, and Convert.ToInt32(object) overhead.
@@ -1041,6 +1060,39 @@ protected override void EmitSetIndex(Expr.SetIndex si)
             return;
         }
 
+        // Float64Array fast path (#878): variable statically typed Float64Array with a
+        // statically-numeric RHS — write the element UNBOXED via $Float64Array.SetUnboxed.
+        // Eliminates the Runtime.SetIndex dispatch, the isinst, the value box, and the
+        // Convert.ToDouble coercion. A non-numeric RHS falls through to the boxed path,
+        // which performs JS ToNumber coercion. OOB faults exactly as the boxed path does.
+        // Evaluate index then value (the receiver var is side-effect-free).
+        if (si.Object is Expr.Variable
+            && _ctx.TypeMap?.Get(si.Object) is TypeInfo.TypedArray { ElementType: "Float64" }
+            && _ctx.TypeMap?.Get(si.Value) is TypeInfo.Primitive { Type: TokenType.TYPE_NUMBER })
+        {
+            EmitExpressionAsDouble(si.Index);
+            IL.Emit(OpCodes.Conv_I4);
+            var idxLocal = IL.DeclareLocal(_ctx.Types.Int32);
+            IL.Emit(OpCodes.Stloc, idxLocal);
+
+            EmitExpression(si.Value);
+            EnsureDouble();
+            var valLocal = IL.DeclareLocal(_ctx.Types.Double);
+            IL.Emit(OpCodes.Stloc, valLocal);
+
+            EmitExpression(si.Object);
+            EnsureBoxed();
+            IL.Emit(OpCodes.Castclass, _ctx.Runtime!.Float64ArrayType);
+            IL.Emit(OpCodes.Ldloc, idxLocal);
+            IL.Emit(OpCodes.Ldloc, valLocal);
+            IL.Emit(OpCodes.Callvirt, _ctx.Runtime!.Float64ArraySetUnboxed);
+
+            // Assignment expression result: the (unboxed) assigned value.
+            IL.Emit(OpCodes.Ldloc, valLocal);
+            SetStackType(StackType.Double);
+            return;
+        }
+
         // Descriptor-driven fast path: when receiver is statically known to be an array,
         // emit direct List<T> access with auto-extension — skips runtime type dispatch,
         // index boxing, and Convert.ToInt32(object) overhead.
diff --git a/Compilation/RuntimeEmitter.TSTypedArray.cs b/Compilation/RuntimeEmitter.TSTypedArray.cs
index 57b9bacc..cb19b51d 100644
--- a/Compilation/RuntimeEmitter.TSTypedArray.cs
+++ b/Compilation/RuntimeEmitter.TSTypedArray.cs
@@ -737,5 +737,69 @@ private void EmitTypedArrayIndexer(
 
         setIl.Emit(OpCodes.Ret);
         typeBuilder.DefineMethodOverride(setter, runtime.TypedArrayElementSet);
+
+        // Unboxed double accessors for Float64Array (#878). These mirror the byte
+        // logic of the boxed Get/Set above but take/return a native `double` — no
+        // Box, no Convert.ToDouble coercion. The IL emitter binds them directly at
+        // statically-known Float64Array index sites, eliminating the GetIndex/SetIndex
+        // dispatch, the isinst ladder, and the per-element box. Like Get/Set, they do
+        // NOT bounds-check: out-of-range access faults via BitConverter/Array.Copy,
+        // exactly as the boxed path does today (OOB semantics unchanged).
+        if (bytesPerElement == 8 && isFloat)
+        {
+            EmitFloat64UnboxedAccessors(typeBuilder, runtime);
+        }
+    }
+
+    // double GetUnboxed(int index) / void SetUnboxed(int index, double value) on $Float64Array.
+    private void EmitFloat64UnboxedAccessors(TypeBuilder typeBuilder, EmittedRuntime runtime)
+    {
+        var getU = typeBuilder.DefineMethod(
+            "GetUnboxed",
+            MethodAttributes.Public | MethodAttributes.HideBySig,
+            _types.Double,
+            [_types.Int32]
+        );
+        var gil = getU.GetILGenerator();
+        // return BitConverter.ToDouble(_buffer, _byteOffset + index * 8);
+        gil.Emit(OpCodes.Ldarg_0);
+        gil.Emit(OpCodes.Ldfld, _typedArrayBufferField!);
+        gil.Emit(OpCodes.Ldarg_0);
+        gil.Emit(OpCodes.Ldfld, _typedArrayByteOffsetField!);
+        gil.Emit(OpCodes.Ldarg_1);
+        gil.Emit(OpCodes.Ldc_I4_8);
+        gil.Emit(OpCodes.Mul);
+        gil.Emit(OpCodes.Add);
+        gil.Emit(OpCodes.Call, typeof(BitConverter).GetMethod("ToDouble", [typeof(byte[]), typeof(int)])!);
+        gil.Emit(OpCodes.Ret);
+        runtime.Float64ArrayGetUnboxed = getU;
+
+        var setU = typeBuilder.DefineMethod(
+            "SetUnboxed",
+            MethodAttributes.Public | MethodAttributes.HideBySig,
+            _types.Void,
+            [_types.Int32, _types.Double]
+        );
+        var sil = setU.GetILGenerator();
+        var bytesLocal = sil.DeclareLocal(typeof(byte[]));
+        // var bytes = BitConverter.GetBytes(value);
+        sil.Emit(OpCodes.Ldarg_2);
+        sil.Emit(OpCodes.Call, typeof(BitConverter).GetMethod("GetBytes", [typeof(double)])!);
+        sil.Emit(OpCodes.Stloc, bytesLocal);
+        // Array.Copy(bytes, 0, _buffer, _byteOffset + index * 8, 8);
+        sil.Emit(OpCodes.Ldloc, bytesLocal);
+        sil.Emit(OpCodes.Ldc_I4_0);
+        sil.Emit(OpCodes.Ldarg_0);
+        sil.Emit(OpCodes.Ldfld, _typedArrayBufferField!);
+        sil.Emit(OpCodes.Ldarg_0);
+        sil.Emit(OpCodes.Ldfld, _typedArrayByteOffsetField!);
+        sil.Emit(OpCodes.Ldarg_1);
+        sil.Emit(OpCodes.Ldc_I4_8);
+        sil.Emit(OpCodes.Mul);
+        sil.Emit(OpCodes.Add);
+        sil.Emit(OpCodes.Ldc_I4_8);
+        sil.Emit(OpCodes.Call, typeof(Array).GetMethod("Copy", [typeof(Array), typeof(int), typeof(Array), typeof(int), typeof(int)])!);
+        sil.Emit(OpCodes.Ret);
+        runtime.Float64ArraySetUnboxed = setU;
     }
 }