did you do something before hand or install anything as of recently? if so it might be running in the background
also there might be something eating up your cpu, check task manager (ctrl + shift + esc) and check whats eating up ur computer. close any background task that you know you don't need (for example, i always close adobe creative cloud and steam when im playing minecraft,) but make sure that you KNOW what it is, if you're not sure if your computer can run without that, then don't close it.
often times, when im playing on vanilla, i use optifine. i'm assuming you've already tried optifine and there are a few settings that you can change to optimise things so hopefully they'll be better. I turn down my biome blend to be 3x3/none, everything to fast, and turned off stable fps.
if none of those solutions work, then it might just be a problem with minecraft itself, and not your computer or your game. minecraft is not optimised, so even on fully capable computers, there will still be lag.
i hope you fix your issue soon! i get the frustration with this type of situation :(
