It's not the monomorphization itself that takes a long time, it's the LLVM codegen due to all the extra code being generated from duplicated function implementations. I've heard macros slow for largely the same reason, it's not the macro execution itself that's slow, it's compiling all the generated code.

If that were true, the same issue would apply to hygienic macros, yet it doesn't. It's distinctly a problem with proc macros.

IDK about that. A project I'm working on is template-heavy in a major way, entirely header-only (~15kloc of this: https://github.com/celtera/avendish/blob/main/include/avnd/c... more or less) but with a barely correctly set-up dev environment with clang and PCH (a couple lines in CMake), ninja and mold, individual rebuilds are pretty much instant ; a complete rebuild which on my machine builds 49 libraries and 38 executables takes a whopping 7 seconds, CMake included.

I read about how .Net does Generics at the VM level and was very impressed, a good compromise between C++ "macro expansion" and Java "type erasure", both of which seem extreme.

At the end of the day, though, when generics are present I stop worrying and just use List<int>, List<float>, List<char>, etc. without a second thought. The compiler, VM or runtime can deal with it although I might get punished in some way.