That depends on your PC hardware.
I have a Ryzen 5600G, which is a relatively modern processor, It also has quite capable video hardware, but nothing extraordinary. I once drew a footprint with an array of 200x200 pads (so 40000 total) and I did not have performance issues as long as the footprint was on the PCB. When you move the footprint, then performance was less then desirable, but still workable. I think 300x300 also worked but 400x400 was too much. But only 2400 pads should not be a performance issue even on an old PC. You can easily try this yourself. Just draw a footprint with a large array of pads and put it on your PCB.
But more important: Do not draw real via’s or pads for your “background”. If you want it to be a background, then only draw something graphical, so it’s just an image as a guideline. And for images, there is no netlist data nor do clearances have to be calculated, so there should be no performance issue whatsoever.
Just yesterday I saw a request on gitlab for more then 32 copper layers. Someone was importing a design into KiCad, but it did not have enough layers.