Good point and thanks for the hint regarding the SSD!

So the main symptoms are:
- loud cooling system, certain areas around the keyboard getting really hot while running a game, eventually getting very slow and (but not always) crashing.
- task manager often (but not always) showing a near 100% utilization of drive "c" even when just "idling".

I have included a screenshot, hoping that it could be useful for the experienced eye.

I suspect two things.

The overheating is caused by some background programs running all the time. I would suggest wiping your hard drive and reinstalling Windows 10. Install nothing else and see if the overheating subsides. Use your ASUS CD to load all the ASUS programs and drivers, then let Windows update them as required. Then reinstall one program at a time and check for overheating after each installation.

Certainly switching to a SATA SSD would help load things faster.

As old as this laptop is I would guess its never been apart and the CPU and GPU be repasted and the fans cleaned out. As old a laptop this is there is going to be a good chance some plastic parts will break upon disassembly/reassembly. Take great care.

Exactly what games are you playing on this laptop?

Thank you both for your feedback. Highly appreciated.

Regarding the background programs, good point: a couple of months ago I kicked out every single program that was of no use for me. I even managed to get rid of Cortana.
That definitely increased the performance.

I did check for malware thoroughly. Until then I was using Trend Micro as an AV but a friend of mine recommended to rely on Defender only, because sometimes the two in combination may lead to conflicts. After uninstalling TM, again the performance got better.
Curious to hear your opinion on that.

Which games am I playing? Now mostly steam related ones, such as ABZU, keep talking and nobody explodes etc.,

I think, that cleaning the interior of my laptop is out of my league. I'm going to check how much this is going to cost me when done by a professional.

One more thing: do you think, this laptop may live some more years if I manage to fix my issues/manage to get back to a decent performance? Or is it in general just too old...

For completeness, I have included another screenshot from task manager.

Looks like you found some relief from some background programs running. Good for you!

ASUS G74S, working properly, can still be used to lightly game. Certainly it can not handle the latest games but it still has life.

Get your laptop serviced, CPU & GPU repasted, fans serviced, upgrade your WiFi Board, install a new SATA SSD drive, and install a new battery. Your laptop could last a few more years.

Good Luck

That is precisely what I am going to do!

Thank you