I thought so... actually my laptop is making a lot of fan noises, but when I took it to an expert he said it was nothing important, eventually the thermal paste to change but he said it didn't recommend it because it would not affect anything...
Also it was fast with w10 and zorin 15 lite.
For your question: It's 1.9.1
WOW!
I make it a point to avoid ever talking bad about another persons mechanical suggestions or opinions.
But...
(cough)
Thermal paste is Crucial in cooling.
If the Fan Management is what is at play, Windows and Z15 may have been utilizing Better Fan Management with faster fan speeds. It is possible that the fans are going out- and the symptoms are now becoming apparent.
It is also possible that the fans are not being properly managed.
So, suggest checking Software Side- first.
Does
sudo apt install lm-sensors
Show it as installed? If not- install it and
sudo sensors-detect
follow All The Prompts to configure.
Run
sensors
Check fan speed:
sensors | grep -i fan
after this... I forget. Im gonna have to look it up,
hang on..
Sorry I hope it's readable...
Optimistic machine... high 100? Wanna sell it? 
Also this can help a lot, too:
sudo apt-get update && sudo apt-get install tlp tlp-rdw
Yeah I smirked at the high 100 too 
Trying the commands brb.
EDIT: The command was successful. Not sure what it did but apparently now when playing a video CPU usage is still low. (Not 100% sure tho because it froze after a moment again )
EDIT 2: After an other hard reboot it seems the cpu usage is still rather high, even if slightly less... (wow and for just a second the temp went up to 98°C!!!)
EDIT 3: After some time the fans started to calm down and now cpu is around 60°C
So apparently normal temperature is lower now (~65°C) but if I play a video or open my browser the cpu usage goes to 100% and temp 80°C.
I think this may bring us back to here...
Do you get any output of
nvidia-settings
are you able to disable Nvidia and test?
Nothing, terminal says not found but can be installed. Should I?
sudo apt install nvidia-settings
Done
What's next?
I have no idea. 
I have never actually used Nvidia Card on a Linux distro. The only thing I do know is that in the Nvidia Settings, somehow, you can turn the Nvidia card off - or set it to not come on...
Would like to test if the Intel dedicated graphics shows the same symptoms with no Nvidia interference.
The only other way to be sure would be to
sudo apt remove --purge nvidia*
Then reinstall with:
sudo ubuntu-drivers autoinstall
edit: I think
prime-select query
Used to show which card you are using - not sure if it still does or requires nvidia prime or ---
Then select card with
sudo prime-select intel
or
sudo prime-select nvidia
Done, at the end it says: Info: the nvidia profile is already set.
After a logout everything seems a bit smoother and the video only takes 80% of the cpu now, so does brave.
I agree that is an improvement; I would have preferred to see cpu usage around 40%...
However, what you describe may be on par with the specs for that machine:
Yes I think it's better than before as well. (Even if cpu rises at 90°C when playing a video -still 60 in normal conditions - at least there's no more freezes and the overall system is way more responsive).
Well I guess there's no escape, I will need to ask the "expert" I know to put some cooling on the cpu when I will get the funds (he told me it was rather expensive because there's a need to remove everything).
Oh btw stupid question but if I try to boot again on the old desktop will the changes be applied on it as well?
There is.
I've done it many times. But... if you are adventurous - and believe in yourself - you can do it. If you take the time to absorb some guides and check the specs- you can absolutely handle doing so. If you want. Someday.
Thanks, I think I will try to see a tutorial on YouTube and if I feel like it I will try to do it myself.
Anyway software-wise I guess that's the best we can do. Thank you SO MUCH for your help. You really helped me to understand and solve my problems.
Should I mark my problem as solved?
I guess as near as it is so far...
I second that.
After I redo the thermo paste of my desktop machine, the idle temperature dropped from 46 to 30.
Oh un autre français!
Anyway thanks everyone, it's really nice to see such a good community!
Hi guys, after several trials and errors I found the solution!
First I reinstalled zorin 16 pro but this time by selecting modern nvidia graphics (note that there was an error but it still worked).
Second when I tried to boot everything seemed to work fine except a tiny bit of lag and the screen which was stuck in low resolution (640x480, so 4:3).
Third I did this (saw it here: drivers - Stuck at 640x480 resolution; - Ask Ubuntu)
$ dpkg -l | grep -i nvidia
$ sudo apt-get remove --purge '^nvidia-.*'
It didn't work when I tried to apply changes at first, so I selected and re-applied the changes again and this time it worked for some reason...
After the reboot absolutely NO lag, my laptop is really fast and average temp of cpu is 60°C. Everything ends well!
Thank you so much for those who helped me, and I hope this will help someone with the same problem!!!