I recently upgraded to Slackware 14.1 (with KDE 4.10.5) and when I launch bitcoin-qt 0.8.5, X windows goes to 100% usage of one core (mouse becomes unresponsive etc) while bitcoin-qt itself uses 1-2% sometimes momentarily jumping up to 16% of one core.

The Litecoin 8.4.1 client causes the same effect.

If I try running bitcoind instead, it quietly sits at 1-2% while X windows stays below 10% CPU usage.

Any idea?