KiCad running slow on Linux

I’m running KiCad on a HP Pavilion laptop with an 8-core processor on Linux Mint.
After a little while, KiCad (all aspects, Eschema and pcbnew) enters “sloth on horlicks” mode, with the cursor struggling to follow the mouse around the screen, the fan starts up and runs full speed, and a look at the system monitor shows one processor running flat out and the others at zero.
A complete shutdown of the computer restores it to normal operation. Restarting KiCad doesn’t.
Any ideas?

What process is consuming the CPU time?

Coud it be related to:

On my Linux box generating that cache file takes about 30s, with one core at 100% and the rest idling. This takes around 30s, but after that things go back to “normal”.

I have not followed up on Seth’s advise to open an issue in gitlab because it is a small thing for me and did not know if others were affected by it and wold rather have developers focus on V6 at the moment.

None of them - they are all zero apart from system monitor.
However, I did notice that Chromium seemed to be occupying a huge amount of memory doing nothing, and KiCad was using 1.3Gb (for a board with 116 components on it, none with more than 3 leads)