CURRENT: extremely unresponsive disk I/O

Since a couple of weeks now, FreeBSD 10.0-CURRENT is really unresponsive
when managing high disk I/O.

This is most noticeable when starting the buildworld process, hwen clang
3.3 starts to build the llvm backend libraries and doing some sort of
portmaster/portupgrade in parallel: even a "make rmconfig config" takes
aprox. a minute to show up.

Is there a known issue at this moment? I can not see such a behaviour on
FreeBSD 9.1-STABLE (similar hardware).

Oliver