15 R3, performance issues

I've been experiencing the following problem(s) with mit alienware 15r3 for roughly the past two weeks (before it used to work fine):

- the cpu is running at lower than usual clock speeds and in particular, it is clocking higher when doing easy tasks such as viewing a pdf or editing a word
document than when under heavy load while gaming; while gaming it rarely clocks higher than 3GHz (whereas until two weeks ago, it would be somewehere
between 3.5 and 4GHz typically)

- the cooling fans are hardly running at all while under load even though the cpu (which is clocking a lot lower than usual) as well as the gpu are running
quite hot.

- And obviously in summary, there has been a sharp dropoff in gaming performance since these issues appeared; e.g. my overwatch fps went from typically sitting
above 200 to below 60 with occasional short freezes etc (with the same settings).

What I've tried so far:
- all driver updates suggested by alienware update
- windows updates
- ran both the prelaunch and the dell hardware diagnostics (didn't report any issues)
- played around a bit with the fan performance options in the bios (which got the fans to run faster, but didn't solve the performance issues)

I also tested how much power the whole laptop is drawing (as it seems like the cpu clocks down as soon as the gpu is needed), and in these tests, the laptop
refused to draw more than 140W (with a 240W charger), is this normal? I unfortunately never tested it before the issues appeared, but the laptop used to also
drain the battery in addition to the charger beiing plugged in while under very heavy load, so I would assume that it used to max out the charger in these
situations.

I would be thankfull for suggestions of whether this is a software or hardware (maybe charger slowly dying etc) issue.