[bluGill]

I'm hopeful modules will come fast, they seem to fix that issue. Modules are already in C++20, but most compilers don't support them well enough yet.

[gpderetta]

I suspect they will help with the parsing, but not with template instantiation cost which dominates metaprogramming.

[pjmlp]

That is what external templates are for, pre-instantiate all common cases in a binary library.