Performance with my singleplayer world

Hello, I am currently having an issue for about 2 weeks that it takes forever to generate my offline 300MB world. I even paid to open a realm for it but it did not work. I have Minecraft installed in a fast SSD and it normally takes 5 seconds to generate the world. When I get in the world, it only loads a few chunks although I have a good PC, tried flying to unloaded chunks and the game froze and crashed. I have tried reinstalling both Windows and Minecraft and it did not work out. Please help