+1  A: 

What platform? I assume from "crontab" that you're on Linux, in which case you can run ffmpeg using nice. That is, instead of:

ffmpeg (options)

Run:

nice -n 20 ffmpeg (options)

This will run ffmpeg at the lowest possible priority, which means any other processing (e.g. web pages) will get scheduled ahead of ffmpeg. It'll still run at "100%" but it'll never take time away from higher priority tasks, which is what you want.

Dean Harding
I tried to run that via PHP but it doesn't even run it :( I've tried the exec(), shell_exec(), and system() functions....it just wont work :( I've also tried to code a script to use a utility called "cpulimit"...and that works for the first process only....grrrr! haha
BoRo
A: 

This worked for me.

brimestone