I have a Dell Inspiron 3521, with a 45 Whr battery and a 90 watt charger.
The laptop runs too slow when charging and eats the battery life like crazy when doing any intensive work.
Initially it used to last 4 hrs with the screen on in 2013, Now it lasts barely an hour, when I am lucky (Doing less intensive work). Meanwhile running chrome with 40 tabs open gives me just about 40 min.
It might have different power saving features, If it is set to performance on battery, but balanced on plugged in, it might exhibit the behaviour you mention.
I tested it in Ubuntu 16.04, Fedora 21, Linux mint 18,
and also Windows 8 (Original which came with my laptop), Windows 8.1, and Windows 10, after upgrading each of them to another,
And then Finally, also WIndows 7 (fresh install).
All the Windows were on Balanced power plan (The default settings), and if I put it on Power saver, it runs too slow Windows 8.1 and down. In windows 10, it runs fast albeit very low battery.