The standalone GC is using custom PAL on Unix: https://github.com/dotnet/runtime/blob/main/src/coreclr/gc/unix/gcenv.unix.cpp

The built-in GC is using PAL from https://github.com/dotnet/runtime/blob/main/src/coreclr/vm/gcenv.os.cpp. This PAL runs on top Win32 emulated APIs that come with extra overhead (extra tracking structures and extra locks). We have observed cases where this overhead is fairly significant.

We should switch the default built-in GC to use the custom PAL to eliminate the overhead of Win32 API emulation on Unix.