I've hit an increasing amount of times when the CPU isn't fully crunching data, due to memory issues.
Mindmodelling, Cosmology, Rosetta... Project doesn't seem to matter much in this, which makes me believe it could be a boinc issue.

It appears when WUs are done, some trace is left in the RAM, causing the RAM to slowly accumulate data that's not really used.
In HTOP it shows as brown data.
*Generic internet image, not my own, to illustrate active vs passive memory data (passive could use garbage collection)

I run a Ryzen 3950x with 32GB of RAM, and the issues have been on both my 3950x, 3900x, as well as my dual core Celeron G series CPU with 8GB of RAM.
On my Ryzen, doing as much as 10 cores out of 32 out of work, due to 'waiting for memory'.
Mind you, the green part in Htop is active or recent memory data, in some cases, doesn't even exceed 50% of the total ram value.
When typing 'free' in Linux, it does show like 200mb free memory, 4GB used, and 8GB total; or a few hundred MB free, with 12GB used out of a total of 32GB memory.

When I restart the service, the same WUs load up, and suddenly there's enough memory to feed the remaining cores.
It's a bug I've only seen from Boinc 7.16.6 or up, maybe in 7.15, not sure, but I never noticed it there. But definitely wasn't present in Boinc 7.14!

Lastly, I tried the setting 'leave non gpu tasks in memory when suspended', but both with it on, or off, the issue still remains... Doesn't seem to affect it..

Seeing that this happens over 3 PCs, using a variety of Ubuntu 20.04, 19.10, and 18.04, I'm not sure if it's related to OS version.

As detailed memory management is in the hands of the application developers I think they would be most interested in your observations. Remember BOINC doesn't look after the memory that any application uses so it is probably worth raising this with the relevant applications.

I run the same processor and 32GB of memory. I don't see that issue with my 7.17.0 custom client. I run 80% of the cpu threads on Einstein and Universe and supporting gpu tasks on GPUGrid, Milkyway and Einstein.

The one application that leaks memory or accumulates "brown" memory on my Ubuntu 20.04 LTS is tracker-extract which is the new Search tool in Ubuntu 19.10 and 20.04. My static RAM usage with BOINC running and the Gridcoin wallets running is around 7.8GB. But if I leave tracker-extract running as default, that rapidly increases to over 12GB and keeps increasing till it eats up all my RAM over several days. This is because of the database growth of the tracker daemon. So I have turned it off and now don't run into a RAM "memory leak".

You might turn off the Search tool in Settings if you are running either 19.10 or 20.04. Tracker is not on by default in 18.04.

Ok, issue was traced back to some bad WUs on MindModelling.
Others are reporting the same issues over there.