A good idea to check the temperature/cpu clock. Alas, the problem evidently can't be fixed with a couple of heat sinks and a fan.
I installed a power monitor on the USB power cord, the Raspberry Pi's temperature monitor and the CPU clock monitor on the taskbars of both computers.
I updated both the Raspberry Pi 2 and a Raspberry Pi 3 to Jessie (uname -r -> 4.1.13-v7+)
and wolfram-engine ($Version -> 10.3.1 for Linux ARM (32-bit) (January 11, 2016).
The benchmark summary is as follows:
Model Benchmark Total Time Voltage Current Temperature Range CPU clock
Raspberry Pi 2 0.011 1248 >5. V 0.3-0.4 A <49 deg C 900MHz
Raspberry Pi 3 0.022 842 >5. V 0.3-0.4 A <50 deg C -
These data are less confusing to me in that the 3 is faster than the 2 running the same versions of the software (to the extent arranged by $sudo apt-get install). Evidently, there's no quick fix.
Finding whether the 3 is going to be usable for my projects will have to wait. Doing linear algebra on the GPU wold be a worthwhile step though.
Cheers,
Fred Klingener