Did you ever find a solution to this problem? I'm also experiencing very low restore rates, less than 1MB/s.

The unrestored log has only grown by 94MB in 8 minutes, but mirroring just isn't keeping up.

We're running SQL 2012 RTM. We've got a gigabit link between the 2 servers which isn't being touched. Data seems to have been sent to the passive server but its just not applying the transactions to the databases quickly enough.

Also checked disks and only seeing 9MB/s writing to the disks, and they're capable of much more obviously.

Darren