If I restarting the system the cpu efficiency goes down to 5-15%. I updated Linux (Version 11.04) and installed 2 packages (libstdc++5, freeglut3) before, but it brings nothing.

The bolded system is exactly what I'm running and I've had zero errors on over 50 tasks thus far. It's not Linux or the CPU causing your problem.

The variability in DSFL task runtimes has been way too high for me to try to make any comparisons. If the runtimes settle down later on, I may try.

Going by what Uplinger said and others observed in the Looong running WUs thread, there have been changes in task length, and there will be more.
Until we know for sure that they have settled on a size range and that WU length is the same for both Linux and Windows, there is not much point making performance comparisons.
As for why, I'm guessing the techs are trying to make the tasks returned stats and ranks more meaningful. I think it also helps Boinc if tasks from the one project are the same or similar length.

So, in a few weeks it might be worth another look.

Since badges are based on CPU time, the fact that windows may run the same work unit a little longer is a plus, in my opinion.

My main concern is why some work units run with terrible efficiency. e.g.



C2Q6660 is an Intel Q6600 running fedora 32-bit with the fedora-issued BOINC 6.10.45; I have 2 other machines with that same version of fedora and BOINC, but with AMD processors, that I've never seen get 'stuck' with such poor efficiency (~12%) work units.