I normally speedrun with 24 chunks render distance, in the overworld my fps are good but when i enter the nether my fps drop to 20, so I dont know if i can use F3+F to decrease my render distance.

Yes

Yes, and if you decrease it to 13 it works best, and you can see everything in the nether/end you could see with 24 chunks too.

Sorry for reviving this thread but I just recommend pausing the game then changing the render distance as you don't lose ingame time.