Adobe Illustrator does not use the GPU in "Overprint Preview" mode.

[Retro User]

When you enable the "Overprint Preview" mode in the "Separations Preview" section, the GPU is not loaded and the load is kept at 1-2%, while the CPU can be fully loaded up to 100%. This results in delays when processing a heavy file with many elements. But when you turn off this mode, GPU works and loads up to 20%, while the CPU works at 30-35% (much faster work in the program is also observed). I have the "GPU Performance" checkboxes in the settings. I also set maximum performance for Illustrator in the system settings (Windows 11). 
Adobe Illustrator 29.5

[Ton Frederiks]

CPU Preview is generally more accurate, but slower, than CPU Preview, so that may be an explanation.

There are some mentions about this in Uservoice.

https://illustrator.uservoice.com/

[Retro User]

As far as I understand, this issue with disabling the GPU in "Overprint Preview" mode has existed since 2019 and was discussed for a couple of years. And later, probably, people just started using more powerful CPUs and the problem of slow operation in this mode ceased to be so relevant. But the problem remains without a solution.

[Ton Frederiks]

Yes, it does not seem solved. Maybe some started using Darken blend mode instead of Overprint, which separates fine and does not need the Overprint Preview (but it is still needed in Separations Preview).