Performance with my singleplayer world

This has been happening for 2 weeks now. It takes ages to load resources packs although I don't have any installed beside the default one.  When it finished loading into the world, chunks are loaded very slowly despite having a good PC. When I place or destroy blocks, the sounds are delayed or no sound at all. The commands are also very delayed or even not registered at all. I have tried reinstalling the game and the world multiple times and it didn't work. I even paid to open a Realm for it and my friends and I cannot join it saying "Disconnected from server" in spite of having a good and stable internet connection of 10MB/s. It is also very strange that when I create a new singleplayer world, it works just fine.