Time machine Backups slow

It is best I describe my setup first.

I have a mid 2011 iMac and a late 2008 MBP that both backup to Time Machine.

The Backups are on a HD attached to the iMac. The Hard Drive is partitioned, one for each Time Machine Backups. So the iMac backs up directly, and the MBP backs up over my wireless network, and "through" the iMac.

This setup has run fine for nearly a Year under Lion. After upgrading to ML on both machines, the Time Machine Back ups on the iMac are fast as before, but the MBP backups are very much slower than before. As I had swapped the HD in the MBP for a SSD about a Month ago, I wondered if all these changes had caused some problem with Time Machine, so I reformatted the MBP backup partition & did a brand new backup. This took about 36 Hours for 90GB, and the subsequent backups still seem slow, though a little better

I understand the Apple do not encourage this way of backing up, but as the problem did not exist before, any suggestions on a fix, or just a matter of waiting for Apple?