XPS17 9720 - suddenly slow and fan constantly running

I have been using an XPS 17 9720 since summer 2022.

For about 2-3 weeks now, the performance has suddenly - not gradually - dropped a lot. Certain apps and a game have huge delays, multitasking with different browser tabs while streaming is becoming a challenge. The fan is very audible which used to be an exception before.

I am not aware of any changes applied except for the frequent Windows Updates. What is weird is that the same thing already happened once, last year at the start of summer. Eventually it went away without me doing anything, but I do not remember how long it took - I feel it wasn't as much for an issue as it is now. Anyway, this made me think it could be related to room temperature/humidity? At my place, indoor temperature can rise to 29°C (84°F) and humidity to 75% during summer. Now I do not want to read too much into this, as this should well be within the specifications and it also does not explain why the performance drop was sudden, not gradual. But it is a weird coincidence.

Noteworthy is the fact that according to Task Manager, CPU is barely exceeding 10%, RAM is at 42% - so it is obviously not an overload issue.

For troubleshooting, I have tried the following, to no avail:

- cleaned the fans carefully with a vacuum cleaner - there was indeed lots of dust but it is all gone now

- ran Dell Support Assist multiple times to ensure all drivers, Bios etc. are up to date

- in the 'My Dell' Application, Thermal mode menu, tested 'ultra-power' (normally it's always 'optimized') - this made the system a tiny bit faster, but still not as fast as it used to be before and at the expense of the fan seemingly being at max

I am reaching out to the community in hope somebody has advice on what else I could try.