The FOG Configuration->FOG Settings->FOG_PIGZ_COMP is defaulted to 9 which means maximum compression/slowest upload.
It only functions on the upload process.
If this is where you’re trying to “speedup” the images, I’d recommend adjusting this value.
If you’re trying to “speedup” the download of images, then check your networking. Is there 10/100MB limiting that lab down from a gigabit network?