I'm seeing the same terribly slow Time Machine backups since upgrading to El Capitan. My Time Capsule backup was running so slow I started a fresh backup to my local GigE attached NAS and it's also terribly slow too. My laptop still running Maverick is backing up fine and fast to the same devices.
Just 14 KB/sec avg in the past 2 hours !!
11/18/15 9:34:15.726 AM com.apple.backupd[1022]: Copied 371.6 MB of 432.3 GB, 45124 of 4387514 items
11/18/15 10:34:20.332 AM com.apple.backupd[1022]: Copied 375 MB of 432.3 GB, 68045 of 4387514 items
11/18/15 11:34:20.784 AM com.apple.backupd[1022]: Copied 471.1 MB of 432.3 GB, 91576 of 4387514 items