As the chat log grows in length, adding anything new to it gives rise to a spike in CPU usage. For example just saying 'hi'.
To reproduce this get yourself a low rate lifesteal weapon such as the golden blade. And go hit something big. You will see many heal messages as you lifesteal from the creature. Because the rate is 1 you get these once a turn (3 a second, approx). As the length grows, your CPU usage will go up and up. If you stop hitting and then just say something you see again a spike.
Closing the chat log doesn't help because it's still being written to even when it's closed (btw if you want to test this and close it by clicking the cross, you can open it again with ctrl + L)
The side effect of this bug is that low rate weapons such as the golden blade and chaos dagger are effectively unusable. /me thought they'd been tried out in test server by those who suggested the stats for them .. obviously not for long enough ;)
Possible fixes : firstly check if there's some memory bug or obvious mistake in how the chat log is written to.
If not, then make older messages automatically disappear from log so the length is kept sensible.
as a quick and dirty fix for lifesteal weapons perhaps the 'heal' message shouldn't be written to log at all but just the number popping out of user's head is seen?