iPhone 6S Plus here. Phone runs great on 11.2.1. What you're experiencing is definitely not normal and I'm way past 500 cycles. I'm curious to see what would happen if you wiped your phone completely and set up as new.
I wouldn't waste the time with that since he is running a 6s. The "evidence" of the past couple weeks is pointing to the battery. Maybe the 6s plus battery isn't affected since it is larger. One way to tell is use CPUDasherX and see what speed it shows the CPU running at.