The only you can do (right now) is live with it (and you can reduce motion for example). There's no jailbreak in ios 11 currently, and of course Apple doesn't sign the ios 7 firmware.
iphone tend to slow down, when more & more updates installed, but i think the iphone is in prime in two scenarios: Original and first major update. Ex: iphone 4s in ios 5 (original firmware) & ios 6 (first major update). After that, it will begining to slow down (according to several youtube videos at least).
I personally think the most stable is first major update, because it fix several issues when iphone launch, and optimize several apps. Especially the last version of the first major update (i mean for example the last update of ios 10 is 10.3.3).