Upgrading from 1.509.1 to 1.516 updated the bundled cvs-plugin from 1.6 to 2.8.
I upgraded to 2.9 but as far as I can remember 2.8 was the same problem:

After the upgrade the disk load on our CVS server increased by about 10 times and user startet complaining why cvs is so slow these days.
I also noticed a few 1000MHz more CPU-Load on our Jenkins Server.

I now did a downgrade from CVS-Plugin 2.9 to 1.6 which fixed the problem.

It can be seen clearly on the attached graphics.
(upgrade was on 13.6.2013, downgrade on 21.6.2013)
(dont get confused by the last day's data, its so many measure points so its getting filled up).

Unfortunatelly the 1.6 plugin seems not to be 100% compatible with Jenkins 1.516 because its startet generating "You have data stored in an older format and/or unreadable data." messages. So I added this bug as Critical because at some point I estimate v1.6 it will prevent me from upgrading our Jenkins server.