A strange thing I have always experienced, if I get bored of one world, or are testing something in one, and decide to swap to a different world without closing Minecraft, the frame rate plummets massively and the lag becomes unbearable. No matter if I switch back to the original world, the same problem persists. However, by restarting Minecraft by closing and reopening the problem is fixed.

I have always had framerate issues, no matter what I do, what I try, the framerate NEVER remains smooth and consistent, but nowadays it isn't that bothersome, just a jitter every few minutes or so.

I would like to make an edit: the problem also happens when I go and explore afar. For a while it is perfectly smooth, then frames just drop. For my guess, (note I am near computer illiterate) the program loads in the chunks and stuff for memory, normal stuff, but when I leave the loaded chunks don't particularly leave, they remain stored in the memory and chew it up, not even leaving the world will free up this memory, as if it's not doing what it's supposed to.