Will core functions in QGIS exploit multi-threading?

Rendering only at this current time (QGIS 2.6).

Martin and I have talked about some kind of generic threaded process API but it's just talks at this current time.

I think that the answer for QGIS 3.0 may be found in this recent conversation posted in Nabble:

Will Processing in QGIS 3 support parallelization?

Citing Nyall Dawson:

If you're after parallelization within a single algorithm (Eg buffering features using multiple threads) then I'm unaware of any plans in place to handle this.