10000mah and an ARM A53 mean it should last 10 hours or more with moderate brightness settings. It's hard to beat this for a light cheap terminal/browser with plenty of battery if that is something you need.
Reviews I read said 4-6 hours. Poorly optimized probably.
I bet it's software compositing. GPU acceleration doesn't work on those boards last time I checked, so it's probably having to do a lot more work on the CPU. It would also explain why it feels so sluggish despite being on paper more powerful than a Raspberry Pi. You should probably forget about trying to play video on it.
Can't speak for parent, but my T430s lasts for three or four hours, despite longterm heavy battery use. Beware of Lenovo's rootkits if looking to buy from them, however.
ThinkPads never had the rootkits, it was the consumer Lenovo models that had them. Very different culture between those two groups.