My note 4 seems to be slower than normal

My note 4 seems slower than normal even after doing a factory reset and a hard reset ,and also going to safe mode etc but still the same....I have flashed a few phone but the last time was on a friends rooted gs3 about 4+ years ago...so if I flash my stock note4 with latest stock firmware will it void my warranty.... ..my reasoning is I think I may have firmware corruption. .I tried many apps to check hardware and they all said it was fine..don't know if they are reliable..but if I reflash and it's still slow I would know it's hardware or more internal. ...so is there a preferred way I know about odin a little bit etc. ...to me... its like reformatting a PC..they normally run much better.afterwards .... I just don't wanna void my warranty. ..thank you