That's being incredibly unfair in comparison. Old hardware had built in support for sprites, and as long as you kept within the limit the game would run smoothly. It had nothing to do with CPU speed - the machine had hardware support to draw say 8 sprites at the same time so if you were only drawing 8 it was fine, but try drawing 9 and you won't render even a frame a second. Nowadays we're doing an extremely brute force approach because everything is its own object that has to be refreshed every frame - and in a game like sonic mania there is so much animation going on it's insane. In a way it would be easier to do a 3D game than a very complex 2D game like this.
Uhm, no? Even a PC video card from 10 years back can literally render millions of textured polygons (=basically sprites) at a steady 60 fps. Any modern card with programmable shaders would be able to do all the transforms and effects on the GPU with virtually zero CPU overhead at the same time.
There really is no excuse whatsoever for a 2D game like sonic mania to run badly on even low-end PC hardware even with built-in graphics hardware.
Memory bandwidth is the main concern when blitting several layers of pixels 1:1, unless you have some kind of crafty system in place for avoiding overdraw. Although Sonic Mania seems to genuinely use a 320x240-ish internal framebuffer, so my wild guess is that the bottlenecks relate to scaling and post-processing, or possibly some funky driver use causing sync issues.
