Performance issue with large XAML files about corewf

[angelowang]

@dmetzgar We are trying to port our .NET 4.7 application to .NET 6, and also met 10x slower performance when executing a simple xaml (but with out own dll reference), from 500ms to 5s.
Profiling shows it's the same callstack as in this PR: #178.

Although it's becoming very fast on the second run, we could have many xaml files loaded in a single operation, so it will become a big overhead.

In .NET framework, compiling VB script is using VbHostedCompiler, but now it's JIT compiler, and BuildAssembly() takes too much time.
Want to confirm is this really a no-solution issue as you mentioned in the PR?

A significant portion of the performance issue comes from the Script implementation itself. An odd thing about the Script implementation for VB is that it was dropped from Roslyn: dotnet/roslyn#13523 (comment)

cc @lbargaoanu

[chenleo-msft]

We also observed a high memory usage issue when running a large xaml file. After debugging we noticed there are dead Native Memory objects that are not collected which can get up to more than 1GB, this sometimes lead to Insufficient Memory issue if the xaml file is too large.