Pav Lucistnik has been working on ports compilations utilising multiple cores:

Two days ago, I have checked in probably most requested feature of last few years. Ports framework now systematically supports building ports on multiple processing cores. It is achieved by passing -jX flag to make(1) running on vendor code. Of course not all ports handle this well, experimental run on pointyhat with this flag globally enabled turned up shy of 400 failures. Because of that, the feature was designed as a
whitelist. Individual ports need to be enabled, and indeed, fellow developers took on and already started adding required declarations to popular ports like Firefox and others.