Battery draining & Performance Drop

My Lenovo Tab S8 has been showing a remarkable Battery life draining in Past month. I haven't installed or upgraded any App or Services recently. I tried to save more battery life by disabling some power consumption settings. And I did Erase Cache Partition as well. I was able to work even under Battery Saving Mode without any performance issue. But disabling those settings didn't get any performance increase or Battery life increase. 3D games like Mortal Kombat X mobile was working fine on this Tab. With recent issues, I couldn't Run the game, as it takes minutes to load & movements were more like Slow motion. But Today I found something Wonderful about performance. When it plugs in to Charger, Performance is good as earlier. When I unplug the Charger, again it goes back to Slow motion speed. Please can anyone give me some clue to fix this. I thought it was some Software issue & even thought to Reset the Tab. But now it seems to have only Power consumption issue. This happens even for Offline games as well.