huge fps drop

1.15 has some re-balancing done with performance, to better share resources with FPS and with chunk loading. What do you have your render distance set to, and does reducing it help the situation?

16

Again, does reducing the render distance improve your FPS?

Also, can you test in the 1.15.1 pre-release ?

I'll try it later and give you a feedback 

Ok, Update. my FPS is fixed and, chunks are still slow to load.   

I'm playing 1.15.1 pre-release 

Ok. I will close this report for now, as there are other similar reports of slow chunk loading. If you have additional information you want to provide, please add it here.

1 more thing, Why corrupted chunks are still in the game, my world got corrupted again, from unexpected crashes caused by my overheating PC. thankfully I got backups. 

There is no guaranteed way to prevent chunks from getting corrupted, especially when caused by a PC crash.
The last updated had a fix for a game crash that was caused by updating your game to 1.15, if your world already had corrupted chunks.

Update, It still lags