When MMX first came out and there were games that supported it, many reviewers were convinced that it improved 3D performance. As far as I could tell, it wasn't really used for 3D and that the only enhancement was to the audio system. But the placebo effect of "If has MMX thus its better" did stick around for a long while.

An aside, but when SSE came a long that was a real big leap in 3D performance, just as GPU's started to gain some independence. So in about 2010, I tried to fire up Turok 2 just to see how fast it would run on a then modern CPU/GPU setup. It couldn't crack 200fps, however games only a year or two later would fly way past that. Turok 2 came out just before SSE and thus basically ran in purely x86/x87 space, thus the performance gap.