Edit by @cartermp:

This affects desktop F# usage on Windows with VS 15.8. .NET Core is unaffected.

Original issue:

I updated my home machine to dev15.8 community edition from 15.7 and noticed my compile times are way longer than usual, ex: instead of taking 1.3 seconds to compile, it's now taking 5 seconds on a project of mine.

I had a thought that it is not ngen'ed, so I went and ngen'ed fsc.exe manually, and now it's back to 1.3 seconds. I don't believe this is intentional.

Workaround

Manually run one of these (Depending on Community/Professional/Enterprise):

ngen "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\CommonExtensions\Microsoft\FSharp\fsc.exe"

ngen "C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\CommonExtensions\Microsoft\FSharp\fsc.exe"

ngen "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\CommonExtensions\Microsoft\FSharp\fsc.exe"