SalC1 made it seem a lot worse than what it actually is. People just believed him without testing it out themselves. There is a server called Purity Vanilla that runs on 1.16. At peak hours it has 75 people on and it has a queue. The server is always at 20 tps. It rarely gets lower.
I personally saw difference with 1.12 server and 1.13 server on VPS with same stats and 1.12 had no problem with 50+ people, 1.12 had problem with 25 people... One thing I didn't exepcted was turtles - they are absolutly not optimalised (Also thing like kelp, but that was something I expected.) - Later sever upgraded and was able to take a lot more people, but it is just way too worse