Iphone 6 slowed down

Hey guys

I bought my iphone 6 in July this year, and it is really starting to slow down. I don't know why, and I think that it started before iOS 9, and if it's already this slow after 4 months there must be a problem. I should tell you that I always close all the apps in multitasking by habit, and background app refresh and stuff like that are turned off, mostly to preserve batterie life but I've been told that it slows iphones down. I also restarted all my settings because I saw that it could help, but it didn't.

In addition, I know someone who has an iphone 6 and bought it in august, and it is still really fast even though she keeps all of her apps opened. If someone has any solution I will gladly take it because I want this phone to last. Thanks!