Your main concern (because you've "disabled Rosetta") seems to be whether things are running properly, or doing harm to your machines. Nothing you've described implies any harm. In fact, the tasks complete in the 3 hours of CPU time that you've (likely) set (or defaulted to) in your R@h preferences.

I think what you are saying is that "wall clock" time is over 12 hours, but actual CPU time is around 3 hours. So the question boils down to asking why tasks might not be receiving CPU time when they are trying to run. This could be due to other tasks on the machine demanding CPU (as BOINC runs at lowest possible priority, and will yield to other tasks).

It seems fairly likely that with one 8 core machine running in 6GB of memory and the other 8 core machine running in 8GB of memory, that you would see "waiting for memory" as the status of several tasks rather then "running". This causes BOINC to stop giving the tasks CPU time until the total memory of other active tasks comes back down to be within the preferences set in your BOINC Manager for memory. So, when memory becomes constrained, BOINC is not longer using all of the CPUs of the machine (or all of the CPUs BOINC is configured to use).

This likely is not occurring on your 4-core machine because it has 6GB of memory (50% more per core then the other machines).

This thread has a number of ideas and descriptions of what to expect and what actions you might take to help things run better.