Terrain generation causes very slow chunk loading

I originally saw it when creating a "Customized" world type but it has been happening across all my worlds, regardless of when they
were made.

On "Customized" it can take up to several minutes for the world to start generating. The loading screen takes 1-2 minutes to go through and then since there is no terrain, the game drops you to the floor of the world slowly. After the world does start to load up, it sends you to the surface and the "Time" starts looping (it is lagging). There is screen tearing and it takes a couple of more minutes of low FPS and blocks restoring themselves to actually be able to play. During that time the sun clearly jumps back every time the game "updates" the world (1-2 times per minute). These world types are virtually unplayable.

In the first screenshot you can see that the game dropped me to the bottom of the world.

In the second screenshot you can see how much time has passed without the world generating itself.

In the 3rd screenshot you can see that the world finally generated itself and the sun jumped back to "Morning", where it's expected to be considering the world generation.

In the 4th screenshot you can see that the game hasn't finished loading yet as liquid cycles haven't finished and there is a water spring and a lava spring that are not processed yet.

This all comes with screen-freezing for a second or so every 3-4 seconds.

Even after hours, the worlds are extremely laggy and the game takes 20-25s to acknowledge that a block has been placed/removed and it can only process one at a time. Even if I place several blocks, or destroy several, only the first one will be processed, the others will return to their original state (not placed or just undestroyed).

I have allocated 3GB of RAM for Minecraft (out of 8) but the amount of RAM doesn't seem to affect anything.

Edit : The more I play the more I think that it's related to world generation all together. I went exploring on an old world and chucks started loading very very slowly after I reached the place where new chunks had to be made. The loading rate was ~10 chunks per 5 seconds. Even when I got back to my spawn location, the world went on loading with this slow speed and was lagging a lot.

When I relog into the world, the already generated chunks load properly.

Edit 2: If you Save and Exit while the world is in this state it can take up to 30 seconds of no response before the game actually gets you to the Main Menu screen.

Edit 3: If you go on playing in the world at some point it gets so... I dunno what... (clugged?) that even already generated terrain takes 1-2 minutes to start rendering while terrain that you've left behind is always dropped instantly as intended.