I am running BOINC 7.16.5, Widgets 3.0.1 on Windows 10 as app, not as a service. I have set to use local options and set the usage of processors and processor time to 100%. I have a CPU with 4 cores and BOINC runs 4 tasks simultaneously.

When I take a look to the task manager, in details every task use 20-25% CPU. The total usage in summary should be 80-100%, but it is only about 53%. I can also see, that the CPU runs at 1.26 GHz from a base speed of 2.4 GHz. So this seems to be the 53%.

I'm not 100% sure, what this means. Is there a way, to use real 100%? I have tried to set the number of processors to 200%, to run 2 tasks per core, but that does't work; the input only allow values up to 100. Is there a reason, maybe on hardware level, why that can't work?

The exact CPU usage is very much dependant on the application. That said, if one tries to use all the cores for running applications you will find that the operating system keeps jumping in and out to do its own purposes (and this is particularly true of later versions of Windows) - these usages do not always show up in task manager. This has a very significant impact on low-core-count CPUs.

Quite possibly your CPU has entered a self-protective thermal state, and throttled down to prevent damage or loss of accuracy.

In short, it's too hot.

Check temps, check cooling system, clean out dust bunnies.

When I take a look to the task manager, in details every task use 20-25% CPU. The total usage in summary should be 80-100%, but it is only about 53%. I can also see, that the CPU runs at 1.26 GHz from a base speed of 2.4 GHz. So this seems to be the 53%.

Interesting to someone who defenestrated over 20 years ago. In Linux when I look at the processes, each task shows as being 95-100% (of the core it is using. I don't know if a task using two cores would show 190-200%. In Windows does task manager show percentage of total for each task or percentage of the core it is using?

%age of the full CPU, so on a quad-core you're lucky to see 25%.

Unfortunately, my dual CPU workstation died, so I can't check that one.

if you're having one of those Pentium D processors, I'd have to agree what's been said before. Your CPU is thermal throttling.

Yes, the problem was the temperature, there was pretty much dust on the cooler from the CPU. The CPU did run at ~97°C. After cleaning it, now I get up to 1.85-2 GHz or 75-80%. Sadly still not 100%, but an improvement.