Is it possible for Boinc to give Vbox low priority like it does with other tasks? If running a few Vbox tasks, the Windows interface becomes very sluggish. Only messing around with a program called Process Hacker is able to sort it.

I don't know the answer to your question but I do remember that when running tasks using Vbox from Linux, I had to leave at least one core free to avoid the problem you describe. That was running climate@home at a time when CPDN didn't have any tasks for any OS. This was one of the many projects that sprung up and then disappeared, I suspect for good now due to funding problems.

This issue was on the BOINC Roadmap 2020 - and yes, I do know it's 2021 now, and it hasn't happened.

There are two parts to this. First, are you thinking about the priority at which the VBox engine itself runs, or the priority at which the science apps inside the container run? It used to be possible to set the science apps to run at a lower priority than the container (as happens with BOINC itself), but the VBox developers removed this feature from their code. Without their support, it's not easy to see how that would be possible now.

Without their support, it's not easy to see how that would be possible now.

Run BOINC inside the VM in VBox and the science applications should return to lower priority. ;-)

Shall I do that while standing on my head?

You seem to have tried everything else, so why not give it a go. :)

Not sure if it's in windows or Linux.
In Windows you can re-set the process priority in taskmanager.
It'll reset every time VBOX is restarted though.

In Linux you can set the nice from 0 to 20.
A higher nice (20) means it'll allow other programs to work first.
It'll also reset when the program is restarted or reset (or it's PID changes).

If your user interface is getting sluggish, it could also be because of multiple GPU WUS per GPU.
Stacking certain GPU project WUs in a single GPU, may make the GPU run more efficiently, but also, it may introduce screen sluggishness.
I have found this especially to be the case on Nvidia GPUs where 3 to 4 (or more) projects are being stacked.
AMD probably suffers from the same, if you increase that number beyond 5 or 6 (depending from project to project, and WU to WU).

I've also noticed that Ryzen CPUs are more sluggish than Intel CPUs.
This probably because their IF (Infinity Fabric) is overloaded.
You have to know there are 24-32 threads all pulling data to and from that bus ring (is what it actually is) at speeds of 4Ghz.
Granted, my Intel CPUs had much fewer cores. I'm not sure if Intel CPUs would do the same at 20+ threads.

Once I finally got Process Hacker to add VBoxHeadless.exe to it's priority adjustment DB; running 16x Kryptos@Home (boot2dockers) VM's seem to be only as sluggish as any other native BOINC app occupying all 16 threads on that Windows 10 machine. (The trick was adjusting the priority of a WU just freshly coming into RAM and then saving into P.H. DB so P.H. could make the adjustment thereafter on each new VBoxHeadless.exe that was seen starting)

It would be nice if lowering the Headless VM's to idle could be accomplished through BOINC management instead of a 3rd party app.
The trick would be getting BOINC manager to adjust to idle only BOINC related headless processes and leave the user's own VM's alone.
TThrottle maintains a DB of BOINC WU's so it can control heat by pausing BOINC related processes for brief moments.
I do not know if it manages VBox WU's, (I vaguely remembering it did tell VM's to reduce CPU usage) but if it does, then those devs might have a possible solution already coded.

Currently all VM's on my machine will now be idle priority, but it's dedicated to BOINC, so not an issue.

(Sorry, about the late reply. Had health issues the last 2 weeks).

Is that on an AMD?

It's the used 2700x custom build w/ 16GB I picked up couple months back.

Running 16x Kryptos WU and an SRBase TF on the GPU.

The OS responds a bit sluggish but nothing longer than 1 sec after a click.

The only thing I can't get rid of is Windows 10 MsMPEng.exe Windows Defender process that constantly uses 0.5-2% CPU and 350MB RAM.
None of the methods that are supposed to shut it down, work after the last update from late last year.

Shut down every nonessential service possible and some that are locked out (who needs audio or server shares on a BOINC machine?)

I want to get my hands on the Chinese government version that made MS strip out the store and Defender.