Trapped instructions are slow all the time, everywhere and anywhere !
I know they are slow, but depending on where they are called, it really may not matter. Even a thousand trap-and-emulate calls aren't a problem if they only happen when loading a level, for example. You simply aren't going to notice (or care about, even if you do) a few hundred microseconds wasted.
If, on the other hand, you are doing a trap-and-emulate every time you do a perspective correction inside your texture mapping loop, it's going to suck abominably.
Ideally, there'd be no emulated instructions but in practise, context is everything.