I was converting a few books from one format to another when I noticed my GPU fans kicking in. Its a 3090ti
As soon as I start the conversion the GPU kicks in to about 70% utilisation
When the conversion stops the GPU dies down to normal background use.

Wierd

Its the 3D Section
Desktop Window Manager is the process using the GPU. Minimizing Calibre does nothing
you are probably converting to PDF which is done via chromium, although it is chromium in headless mode.
Nope - EPUB to MOBI. Definately not PDF, horrible format
Then it will be SVG rasterization. Convert to azw3 instead which supports svg
Yeah - maybe - but I need MOBI. I store everything in MOBI.
Note that I am not saying its a problem using the GPU for this - if it needs it. It was just wierd when I noticed it
Your Kindle is that old (your join date)?
Mobi isn't as bad as PDF as a source, but it is limited on what it supports.

Down grading newer formats so you can (assumed only) 'store in Mobi' is like only using 78 records (mono) instead of a 33 Stereo LP to listen to Classical Music (uses full frequency range). Once removed (the content features), they are lost.

Look at the code of some of the early files from the Baen Free Library and see the hoops they did to improve the presentation of early PRC and Mobi books.
if you need Mobi because your Kindle is too old to support KF8, then you really should get a new Kindle.

If that's not the case, you do not need Mobi and can forgo Mobi forever.
You only need mobi on K1, K2, DX and DXG.
The superior azw3 works on the Kindle 3 (Keyboard) with a FW update, though the earliest FWs on the KK3 only did mobi.
All later models do azw3.
https://www.amazon.com/gp/help/customer/display.html?nodeId=GKMQC26VQQMM8XSW

Anything not a Kindle that uses mobi will work far better with epub2.