Iphone 8 Plus Slow after iOS12.1.1 update

Hi, i just bought a brand new iphone 8 plus recently when Apple was announcing XS & XR. I bought it a an Apple authorized reseller. It was running fine & quite happy with it. But recently after updating to iOS12.1.1 my 8 plus experience a slight decrease in performance. So i tested it on Geekbench and got a significantly low score for multicore cpu test. Below 105200 which is a standard for iphone 8 plus. Later i purchase an app call cpu-z to do a cpu speed test & get 2064Mhz - 2377Mhz at most. So i want to know if Apple throttling down the speed or my cpu have some problem? It is still under warranty. Please advise. Thanks