It's how fast actions are in the game, how fast mix ups are, how many inputs you need to make in the neutral in a given time and yes how fast you have to make decisions.Do you defend speed as in how many decisions a player has to make or how much shit is flying across screen?
Shit flying across screen has nothing to do with it really but that's a different matter. That would more fall into "how much stuff you have to keep track of at a given time".
I would consider both MVC3/MVC2 and Smash fast games along with Tekken. Most fighting games are fast though... few are legitimately slow. It's all relative really.