Steam Play, aka Proton, is a way to run Windows games on platforms they weren't intended for. It's like an emulator that way. However, it's not really an emulator; it's an alternate implementation of Windows code that a lot of games depend on. It's based on WINE (which stands for WINE Is Not An Emulator).

What do you think of using Steam Play for speedruns?

Idk ask the mods of the games you want to use it for. Something like that has the potential of being inaccurate so you’ll need to check with any mods (and possibly provide some testing you do to show it can be comparable to using Windows).