I had another one of those slowdowns. Switching symbol settings didn't help, and my symbol list was only about 20.
In fact, not just the editor, everything became slowish. Switching to another tab - half a second delay. Moving separator - half a second delay. Switching from projects to symbols - a second delay.
In addition, I started having visual artifacts, a rectangle from another part of the code kept hanging around. To get to this point I kept C::B open for about 2 days without a restart.
Top showed that Xorg CPU usage shot up to 80% and Kwin started showing up high on top as well. Xorg lingers at the top of the CPU usage (in the range of 50% - 80%) for about 10 seconds if I don't touch C::B and then gradually goes down to single digits. Whenever I try to do anything in C::B Xorg CPU usage would go up again.
I have a 7MB video of this slowdown. Unfortunately, the mouse pointer is not recorded, so it is hard to judge the delay from the video.
Edit: my system and C::B version are listed in the previous post.