Putting in 100 Mhz or whatever RAM will not affect your processor speed in anyway. But it will obviously be less in performance (relative) than 133 MHz RAM. The only area where you can really "lose in terms of speed" is where you have another RAM slot filled with 133 MHz RAM - bec. afaik all the RAM chips work with the same speed - the speed of the slowest chip.
That means if you have a 133 and a 100 MHz RAM chips in your two RAM slots, both will run at 100 MHz.