The CPU is an i3-3220 (Core i3 "Ivy Bridge" @3.3ghz, 2 physical cores with hyperthreading). But in STO, the "Multi-core rendering" feature is disabled. I can mouse-over the "On" position but clicking does not change the setting.
Looking in the file "gameprefs.pref", STO correctly identifies the chip as
Prefentry SystemSpecs.Cpuidentifier "Intel64 Family 6 Model 58 Stepping 9, GenuineIntel"
Prefentry SystemSpecs.Cpuspeed 3.292e+009
I wasn't even able to SET the setting, nevermind reboot to make it take effect.
That's what I started to wonder. And maybe that would be a solution if anybody else has a similar problem.
But once I started digging around for cases of multi-core CPUs only being recognized as single core CPUs, I found my problem! Turns out that you can tell Windows how many processors (well, simultaneous threads really, but Windows calls them processors) it has, and it will believe you without bothering to check.
I found the setting by going to Start, Run, and typing "msconfig". On the Boot tab, click the button marked Advanced Options. The first item is "Number of processors". This should the number of simultaneous threads your CPU can support, e.g. a dual core hyperthreaded CPU can support 4 threads, so the setting should be 4. Windows will normally attempt to figure this out automatically, but you can shave your boot times down a few seconds by putting a checkmark in the box and choosing to override the automatic detection. Turns out this setting was on 2 when it should be 4. I cleared the checkbox to let Windows redetect the hardware, and rebooted. Now Windows says I have 4 processors (aka simultaneous threads), and I can enable multi-core rendering in STO.