In Windows there's also a Memory priority and I/O priority. I used an online guide to get those lowered for uTorrent process, and it seems the program become less noticeable when ran in the background.
Screenshot of BOINC spawned process and my adjusted uTorrent process

I just wanted to ask, what is a reason not to apply these low priorities to BOINC processes too?

BOINC its parts (boinc.exe, boincmgr.exe) don't need to run at low priority since all they do is manage which project's application to run next, when work should be downloaded etc.

It's the project's applications which do the work and these are normally run at low priority. Some exception for some of the (OpenCL) GPU applications here and there, and the QCN application, which run at Below Normal priority, as else they don't utilize the full use of the CPU.

So if the project's application isn't running at Low priority, you'll have to ask the project why they decided to do that, in this case at the Einstein project. I just suppose it's because it's still their test app, not a full production one (seeing how the app's version numbers is 0.23), but you really will have to ask them, at their forums.

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Alright, thanks.