After CrashPlan PRO identifies the unique data in a file, it breaks the data into pieces and then compresses each piece. If the compressed piece is smaller than the original, the compressed piece is backed up. If the piece is not smaller, the original data will be backed up. CrashPlan PRO does not compress certain types of files, such as files ending in .zip.
Compression does consume some CPU, but not enough to offset overhead due to maxing out your bandwidth or I / O. Because we tend to have more CPU available than bandwidth or disk I / O, we do not recommend turning off compression.