Unrelated, but about a year ago I downloaded Sonic the Hedgehog 2 from the Xbox Store onto my Xbox one (I think it cost £0.99).

Anyway, it was about 500Mb in size and it runs Sonic 2 on the Xbox One's Xbox 360 emulator, which is then emulating a Sega Mega Drive.

That just blows my mind! 8 x86 cores to emulate 3 PPC cores to emulate a mega drive.

The game is more graphically intensive than you might think and the people that care about this kind of game very much value a solid 60 FPS frame rate.

Works at 60fps on Nintendo Switch though, which isn't exactly the most powerful machine in the world.

Not just works, it's 100% solid on the Switch outside of the mode-7'ish bonus game with no stuttering or frame drops.

It's good but definitely not 100% solid 60 fps according to Digital Foundry's analysis: https://youtu.be/DXturRMOzU4?t=7m23s

I imagine it is easier to get working on a known hardware configuration.

It's about Sega Saturn levels of graphically intensive, and should be well within the range of capability of even pleb-tier GPUs from the modern era.

yet somehow it ran at 60fps 20 years ago on hardware that was 10000 times slower

hmm

The game includes garphics which wouldn't be possible on the Mega Drive / Genesis. For example alpha blending or lots of sprite rotating.

Also Sonic on the Mega Drive had multiple framerate drops, e. g. when you lose lots of rings.

It would've been possible on the Saturn, and on the Dreamcast at 60 fps, easily. How much more powerful are even mediocre modern PCs compared to the Dreamcast?

Saturn would most likely choke on memory requirements, given how much is going on in the levels, as well as lack of transparency support. Symphony of the Night was the state of the art 2D platformer for that hardware generation and it has frequent loading pauses, despite being much more static.

Dreamcast would probably work, given enough effort by people who know the platform inside and out. But then, a much lower barrier to entry is probably a significant reason as to why Sega was willing to fund another 2D Sonic game in the first place.

Though we do have 4K tvs now.

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.

Note that they actually did set the minimum requirement to "a PC video card from 10 years back": http://store.steampowered.com/app/584400/Sonic_Mania/

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.

There's slowdown in some sections on the Xbox One version, too.