diff --git a/src/Compiler/Driver/CompilerImports.fs b/src/Compiler/Driver/CompilerImports.fs
index f0868919ad0..46a3b83b604 100644
--- a/src/Compiler/Driver/CompilerImports.fs
+++ b/src/Compiler/Driver/CompilerImports.fs
@@ -2341,6 +2341,12 @@ and [<Sealed>] TcImports
             let! ccuinfos = phase2s |> runMethod
 
             if importsBase.IsSome then
+                let addConstraintSources (ia: ImportedAssembly) =
+                    // Only an F# assembly can carry a trait constraint to label.
+                    // Prevent force-reading of the whole assembly namespace tree for other assemblies.
+                    if ia.FSharpViewOfMetadata.IsFSharp then
+                        addConstraintSources ia
+
                 importsBase.Value.CcuTable.Values |> Seq.iter addConstraintSources
                 ccuTable.Values |> Seq.iter addConstraintSources