Linux

85C could be throttling

If the CPU clocks are dropping to ~200-300 MHz while the temps are 40-45C (like in the screenshot) then it's not thermal throttling. The clockspeed would go back up when the temps go down. And it would only throttle enough to keep the temps under the desired temp.

I would investigate what performance profile the CPU is using.

There is a tool called cpupower that will list out all the information about the CPU clock states.

I have a Ryzen CPU so the desired governor is going to be different than an Intel laptop, but for example, the output of cpupower frequency-info for me:

analyzing CPU 13:
  driver: amd-pstate-epp
  CPUs which run at the same hardware frequency: 13
  CPUs which need to have their frequency coordinated by software: 13
  energy performance preference: balance_performance
  hardware limits: 600 MHz - 5.76 GHz
  available cpufreq governors: performance powersave
  current policy: frequency should be within 2.98 GHz and 5.76 GHz.
                  The governor "powersave" may decide which speed to use
                  within this range.
  current CPU frequency: 4.39 GHz (asserted by call to kernel)
  boost state support:
    Supported: yes
    Active: yes
  amd-pstate limits:
    Highest Performance: 166. Maximum Frequency: 5.76 GHz.
    Nominal Performance: 124. Nominal Frequency: 4.30 GHz.
    Lowest Non-linear Performance: 86. Lowest Non-linear Frequency: 2.98 GHz.
    Lowest Performance: 18. Lowest Frequency: 600 MHz.
    Preferred Core Support: 1. Preferred Core Ranking: 231.

Which you can see lists the hardware clock range, the current governor's policy frequency range, the actual current CPU frequency, and how it picks different frequency ranges.

I used to use cpupower on an old laptop to force it into the performance governor, because it would not clock up high enough without it. This obviously does negatively affect battery life, but i was plugged in most of the time anyway.

But either way, look into cpupower for determining the governor/power profile and also figuring out which governor you should actually be using.

Yes create a Windows stick and boot from that. But that's a lot of work, just create a Windows setup stick and install it. Installing Windows and running some tests should take less than an hour and can save you a lot of headache. You can download the ISO directly from Microsoft and you don't need a license for some simple testing like this.

Temperatures read out in software can be very unreliable. The software might say it's perfectly fine and the thing still throttles because some part of the chip gets too hot.

Cleaning it out and re-doing the paste is my go to for any refurbishment on old laptops. Unless it's something weird like liquid metal or something, it's usually very easy and quick to do. And even if it isn't strictly needed, it usually helps.

CPU thermal throttling - any idea on how to test (or even fix) this?

Check for tons of pet hair stuck in the fans or the vent paths. Seriously.