Ever since I started borging, and after running FAH for months on my machine, I was convinced that - just like SETI - it didn't interfer with other apps/games.
But to my surprise there were some complaints:
- a friend w/ a XP2000+ and 512MB RAM had problems playing some games online, one even refused to connect to any server. Turning off FAH solved the problem.
- another friend w/ a XP1900+ and 512MB RAM reported significant slowing while playing some games, and even while running other apps. He also tried turning off FAH, then back on, etc. to make sure.
- my brother w/ a Dual XEON 500 and 1GB RAM, while using both processors, would have trouble connecting to (dial-up) internet. Very strange, but solved when running FAH on only one CPU. He also reported that FAH slowed the PC a bit, especially while clicking things like "Start" button, etc., there was a slight delay.
All other people only complained about the increase in CPU temp. lol, but did not report any change in speed, while just using "normal" apps or playing.
I have not tested gaming performance vs. running FAH on my computer - mainly because I always leave FAH on - but as far as I can tell there's no difference. I have 1GB RAM with no pagefile though.
So I guess it can depend on the system configuration and app/game, there's no telling that FAH will or won't interfer with other programs.