It's a normal system operation.
When gaming on laptops you should plug in into a wall outlet for high performance power requirements.
Most laptops enable "power saving" features when on the battery to prolong battery life.
the thing is i also have another laptop, its hp with intel i5 processor but it goes to full cpu speed on high performance under battery only. so i was thinking if acer nitro 5 laptop wants to save battery why not stay at its base clock frequency which is 2Ghz instead it is @1.6Ghz on battery.