I was running Indie Velo on a M1 MacBook Air and 27” 4K monitor. I think I had the graphics settings at 4K, but it was stuttering badly at times. The graphics themselves didn’t look very impressive either. By contrast, Zwift graphics look great and run super-smoothly at highest resolution on this same MacBook. No contest there. But then Indie Velo is still free and has a lot of options to play with.
Zwift isn’t perfect either. Drafting is still a bit iffy and I don’t like the way it hangs your power for a few seconds when you coast or soft pedal. But I think it does still offer the best overall riding sim, especially for group rides/races. Rouvy is probably the best alternative for real world route simulations.