I tried to load one of my worlds that I had saved in 1.15.2 and it took 8+ minutes to load. When it did load, the chunks didn't render for 2 minutes and then one of my builds showed up and windows gave a not responding error. I have tried it with 2gb of ram allocated, 4gb and 6 gb. By the way, I originally saved the world in 1.15 a few months ago.