iPhone 6 Plus variable performance (not battery performance)

I have an iPhone 6 Plus, running IOS 11.3.1 and i have been experiencing slower performance across apps since I updated from 10.3 Which I expected. My battery is at 99% and is at “peak performance capability”. At certain moments however, my old IPhone seems to be running really snappy. App load times are much quicker and even games like NFS: No Limits, which exhibits much lower frame rate and extreme stuttering after the IOS 11 update, magically runs like a charm. High frame rate and no stutters. This behaviour would persist for a few hours (1-2) until it’s back to its old state. The stuttering problem of the game mentioned above, indicates low amounts of available RAM (yes it has 1GB, but why is it enough at certain times). And lower framerate would indicate a running process hogging the SOC. Before I run the game I always perform a memory clear (power off screen + home button hold), so user applications running is not the case. Can someone throw some light into this. I feel there is a light in the tunnel for this old device and I want to find out why this performance comes and goes.