That depends on the cameras you use and also on the tools you use in RawTherapee.
The decoding of most raw files does not make full use of all cores. The decoding times also differ a lot. For example Canon CR2 is one which needs more time than Nikon NEF. To get short decoding time in RT, you need higher clockspeed.
After the decoding pass most of the processing pipeline makes full use of all cores. One exception is the Tonemapping tool, which makes not full use of all cores.
I can give you more details if you can tell us which tools of RT you use.