I have 4 CPU cores. So, like possibly many other helpers, I have excess CPU capacity when I am using my PC, as most of what I do is not very CPU intensive.
At the moment I have suspended all bar one task, set "Run always" and it does it's work in the background, never exceeding 25% of CPU, as expected. This goes completely unnoticed by me.
However, when I walk away for longer that the Idle trigger time, that's still all it does, because of what I have set up.

So my suggestion is for an new option in Preferences that allows 1, maybe 2, tasks to work at all times, but still retain the traditional current system of running as much as possible when inactive. This new option, which could be called "Tasks to run always", should also allow a 0 (zero) setting, disabling it.
I would be quite happy to have one task running permanently, whilst still maintaining the "screensaver" style preference we have now.

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FWIW even on my single-CPU systems I have BOINC running all the time, and I never notice it, either at home or at work. When I monitor my CPU usage I see the science apps' share drop whenever the system or other applications show increased activity. So IMO—and assuming adequate cooling—there’s no need to place further restrictions on BOINC’s demands than its “nice” behaviour already does. And there’s always the “snooze” function, if you occasionally do something particularly CPU-intensive.

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