Hi,
When RPR mode is used, the standby supervisor engine partially boots and keeps synchronized copies of the active configuration, which shortens the time needed to bring up the standby supervisor engine and have it start handling traffic from 1.5 minutes (for a cold boot on the standby) to 30 seconds (to finish the boot and re establish links). Hot standby applies to SSO mode. If possible use SSO instead of RPR.
HTH
Reza