You can reduce pbackup system load in
ispmgr.conf configutation file
BackupPrior - priority for pbackup and all associated processes. You may use values 0 to 19. The larger the number, the less priority a process has. The default value is 0.
BackupIOClass - pbackup I/O priority classes. You may use values 1 to 3, where 1 is a real-time priority level (realtime), 2 is an average level (best effort is set by default), 3 - a minimum level (idle). Used only for Linux-based operating systems.
BackupIOClassData - pbackup I/O priority classes. You may use values 0 to 7. The smaller the figure, the higher the priority. The priority is set only if the priority class is set to 1 or 2. The default value is 7. Used only for Linux-based operating systems.