I created a new world (World type:Infinity, Chunk stimulation:4 chunks, Mob Spawning: false, Gamemode:creative, is single player)
At the beginning the FPS was 60 (little high or little low) and was very smooth to play.
I started flying around and generating new chunks, I kept on flying like 3 hours.
As new chunks were generated the FPS started dropping. I then save and quit the game and opened the world again after a while and the FPS was even worse than before.
I did not do anything exept of generating new chunks. There and no resources pack or behaviour pack attached to the world. I wanted to post the world here in this report, but I was unable to.
I highly requests the minecraft developers, to remove this issue to the minimum, of FPS dropping.