Is it just my imagination, or have others seen it too?

Yesterday I updated from binaries based on a Nov 24 cvs update to
source based on Dec 10, and now my system is once again *very*
unresponsive when I'm (for example), doing a tar of a directory tree
into a 450MB tarfile (no compression).

This is the way things were until a couple months ago, when they
straightened out quite nicely, maybe even a bit worse. Before, the
only problem was that emacs would become *completely* swapped out and
take forever to swap back in; now even my mouse pointer is several
seconds behind mouse movements (I'm running VNC as an X server,
displayed remotely on a Windows PC).

Has something changed, or am I just a hypochondriac?