Its misleading to call it "cost". In the C++ world only runtime cost matters. If using reflection allows to generate faster result code, it doesn't matter how long it takes to compile.
I'm now trying to migrate from msbuild to cmake+sscache+PCH for std libraries while also trimming unnecessary includes to reduce suffering in the future - if not for me then at least for future developers. So I would say compile time is important for development. It causes other limitations too (like bugfixing becomes a huge commit with several squished fixes together to avoid recompiles, messing up git history or slower context switching when developing several features in parallel)
I'm sure you wouldn't say "it doesn't matter how long it takes to compile" it if took days. So where do you draw the line? Regardless, it matters.