"__fastcall was introduced a long time ago. At the time, Watcom C++ was beating Microsoft for optimization, and a number of reviewers picked out its register-based calling convention as one (possible) reason why.

Microsoft responded by adding __fastcall, and they've retained it ever since -- but I don't think they ever did much more than enough to be able to say "we have a register-based calling convention too..." Their preference (especially since the 32-bit migration) seems to be for __stdcall. They've put quite a bit of work into improving their code generation with it, but (apparently) not nearly so much with __fastcall. With on-chip caching, the gain from passing things in registers isn't nearly as great as it was then anyway."