The code is quite heavy on compile-time and could use some optimizations (e.g. every enum_cast takes 1s to instantiate in my test).
clang got improved recently to cover constexpr evaluations in time traces which will come in handy: https://reviews.llvm.org/D136022.

Half of the parsing time alone is spent on constexpr_switch for example, and is_flags_enum is very heavy to instantiate because it calls values.
Here's the test-cpp17 time trace generated with -ftime-trace -ftime-trace-granularity=100, can be viewed with https://ui.perfetto.dev.