Sounds like you're using a low power adapter. I have a few ipad2 that the kids use for things. Even if they have 1% and we plug them in they can be used while they charge.
I'd also suggest lowering the brightness slightly, can make a huge difference to battery drain
It was a 10W. I ran many system-intensive games and only remember battery drain when using a smaller power brick, but never ran Geekbench on loop to test that.
I'd also suggest lowering the brightness slightly, can make a huge difference to battery drain