My iPhone 4s has been very slow now-a-days. Any ideas on how make it faster and better?

Let me tell you that less than 10 MB storage is available. I know that must be the major reason for iPhone to be slow up to this extent. But will it be faster enough when I clear my iPhone memory? Is it gonna happen for sure? Moreover, I'm currently running iOS 7.1(11D167) on my iPhone… Is that good. Or should I upgrade or downgrade? Also tell me how to downgrade iPhone. Which is the perfect iOS for iPhone 4s.

IOS 7 is still the best for 4S, my iphone 4 using iOS 7 too and yes, they are kinda slower compared the iOS6.

but i don't think having a little space left on phone affect the performance of iOS.
as for 4S i will recommend iOS 7.1.2(which is the latest for iOS 7).

btw what did you mean by slow? Lag when opening apps? Unlock? Making a call?