Hmmmm,

I'm using Windows 7 64-Bit, without PC Suite.
I've just conducted a test copying a folder containing 4*450MB Videos
Results are comparable whether the N900 is switched on or off:

Write to Internal Flash ("N900" partition) == 10 - 11 MBs
Write to External microSDHC == 5 - 6 MBs

Read From Internal Flash == 16-17 MBs
Read from External microSDHC == 9 - 10 MBs

FYI - the microSDHC is Class 2 Sandisk so I'm not expecting stellar behaviour.

As far as I can see, having the phone on or off make no diff.