The reason it lags is because it's writing pure, raw, uncompressed video data to your HD as it happens, simply put, and unless your lag is MAJORLY NOTICEABLE that's normal to see a little slowdown; s'why fraps' files end up being 3gb for 3min of footage. It primarily depends on your processor and your HD's speed, and multiboxing on a single machine multiplies this effect because the machine's already taxed a fair bit. On my old comp when I fraps'd heavy fights like the skeles spawning on Nef, it literally became a slideshow with a 10sec+ delay even though I still heard the sounds; I upgraded my comp and bam, I barely notice a hiccup when I start to record but to the trained eye I still notice a difference. I tried fraps'ing my ZF temple event run(will be happy to link that if anyone gives 2 shits, that's the only reason I Haven't shared yet lol) and it was a pretty noticeable, but still more or less playable slowdown.

On that note, DO NOT ALT TAB WHILE FRAPSING. In the same ZF vid I mention above, I missed the entire end fight vs nekrum/troll priestess then the human/dwarf/orc npcs because I alt-tab'd to cast Hurricane on the alt boomkins, and apparently alt-tab to fraps = "STOP RECORDING NOW DAMNIT" even though my record start/stop key is F10