When compile a large application that uses WinForms there are many classes that derive from System.Windows.Forms.Control/Form and have class hierarchy depth around 12-13 levels deep.

This hits a code path in ILC type system that doesn't scale well:

Bulk of the time in the marking phase (several minutes) is spent in Internal.TypeSystem.MetadataVirtualMethodAlgorithm.ResolveInterfaceMethodToVirtualMethodOnType(MethodDesc, MetadataType). It recursively calls into itself through ResolveInterfaceMethodToVirtualMethodOnTypeRecursive. ResolveInterfaceMethodToVirtualMethodOnTypeRecursive also walks the class hierarchy upwards. This results in an algorithm that doesn't scale well with the class hierarchy depth.

Perhaps there's a way to short-circuit it to prevent double recursion? Or maybe the algorithm can be rewritten in more efficient manner? Dynamic programming? I am open to ideas.

Fixing this is likely to cut the compilation time by several minutes for this particular application. I expect it to affect every WinForms application (once the scenario is supported) in one way or another.