Well, I have a gaming laptop I bought quite new, but second hand, that was almost never used without cabel (I saw the formerly setup) - and the batterie was allmost dead and blown up in size after some months of use. But since it is a gaming laptop - it generated massive amounts of heat under load. I bought a new battery and took care of the heat: and it is still quite good, after roughly the same time and amount of use and I do charge till 100%.

Throttling of CPU and GPU should surely account for 90% of what you describe, coupled with an old, uncycled battery that can't move ions no more.