We are having the very same issue.
actually now we're testing bews2012 as a candidate for production.
DAG Exchange 2010 (2 db servers, 2 cas servers) all fixes are up to date
bews2012 all patched up
agents are also up to date
backing up to raid-5 de-dublicated disk array on windows server 2008r2 with more that 100MB\sec write speed using 1Gb link.
the full backups are seems to be at normal (at least acceptable) speed, but incrementals are disaster !
having 1.7TB total size (4 DBs) incrementals are at speed 10-30MB\sec on updating catalogs state.
Waiting very much for a patch.