In a streaming replication can it be assumed that if both primary and standby are of the same hardware, then the rate at which transactions are applied on the standby will be same as that on primary. Or standbys are always slower than primary in applying transactions because of the way replication works.
It could go either way. The standby only has to apply the changes, not compute them, so if the primary does something like:
UPDATE foobar set col1 = (<slow select query returning one row and one column>) where col2=? ;
then the standby will replay it much faster than the primary needed to execute it.
On the other hand, replay is done single-threaded. If the primary has a lot of active concurrent connections, replaying them serially could be much slower than it took to produce them in the first place. This might be true of both CPU and of IO. If your IO is a big RAID system, the primary could keep multiple spindles active simultaneously by having multiple connections waiting on different pieces of data independently, while replay will wait on them serially. There is currently not a prefetch mechanism for replay.
