I was having this problem after upgrade to 10.7.5, thought it a Time Capsule failure and was getting ready to replace the drive in it when I found similar forums that said problem fixed in Mountain Lion so I upgraded. Not fixed.
Tried complete Spotlight index rebuild. In fact, I deleted the spotlight index directory and left spotlight disabled when I upgraded to Moutain Lion. Turned it back on after upgrade, 73 minutes to reindex 293GB. Home free I thought, but Time Capsule backups still only processed data at KB speed to both Gigabit connected TC and USB connected 1 TB external drive.
I finally solved this though. I run Sophos Anti-Virus. The Time Capsule is at work and the external hard drive is at home. MLion Time Machine supports two devices so I have them both setup. On Sophos' Preference Pane, On-Access Scanning icon, Excluded items tab, I dragged the disk icon for the external hard drive to the Excluded Items list and then forced a Time Capsule backup (backup now) and when that disk mounted, dragged the icon for it to the Excluded Items list, too. Time Machine backups to both devices are completely back to normal!