Gulp runs in node, which runs single core so compilation speed will be effected by single core speed. I'm not really that familiar with how "Parallel" msbuild/roselyn is these days in regards to core usage. I'm pretty sure Roselyn/Visual Studio does some very clever stuff when compiling your code. I think it keeps a shadow copy of your on disk files and only recompiles the AST sections it needs too. I remember watching a talk on how Anders reimagined the .net compiler to approach compilation this way. You should probably look at other multi-core workloads you might be doing on the dev machine that would benefit from the increased core count.

Afaik, we're using gulp with some paralellization plugins.