Processing windows→Checkbox: lowest process and I/O priority

In the processing windows, add a checkbox to put the Audacity process and its I/O into the lowest priority mode. Back to their previous priorities when the processing is finished.

Example of a processing window attached.
Processing windows – Example.png

What would be the benefit?

If people use Audacity but do other things (Internet, videos, …) during the processing, if their computer has low performances (like 1 or 2 processors + bad HDD) they will have slowness problems (worse if they are ordinary people because there are great chance they close Audacity or their other software).
→ so during the processing, having the possibility to choose the lowest priority for the Audacity process and its I/O requests will greatly reduce up to fix this problem + significant time saving for those who know how to change that (= no need to change the priorities, then change them again when the processing is finished, and so on)

I forgot this: check the checkbox by default is even better (which should refer to a setting in Audacity to set it by default or not).

It could also be argued the other way, that if export is performed with low priority, exporting will take longer and it will be less evident to the user that exporting is still happening, thus increasing the risk of unwary users shutting down their computer before the export is complete.

Yes but your example is much less problematic. It is the problem of the user if he turns off his computer before checking what he has launched, intentionally, is finished, and he has no excuse if there are visual indicators (and Audacity has a visual indicator—at least in its window).

In comparison, the priority system is not known by at least ordinary people, and is time consuming to manually manage for others.

Among my computers, I have one of this kind. And for example if the HDD is requested by at least two software (Audacity + Firefox for example, and I use Firefox while Audacity is processing), it is simply painful.