Since you mentioned cron, I assume your referring to Reseller level or sysbak backups.
Depending on the ammount that must be backed up and compressed, the load in general will varry. Normally on a good server, no other services should be affected critically. There may be noticeable slow downs and so on.
The magnitude of noticeability will also depend on the load of your server before you initiated the backups causing a surge of usage.