Speed comparison between Grunt and the aforementioned Gulp
All Gulp plugins are just duplex streams that read in data and output data. Everything can be processed in memory, with the output of one stream piped as input to another. Much like Unix pipes.
This gives Gulp a huge speed advantage over Grunt, because I/O is very expensive when compared to in-memory operations. On top of that, Grunt has to compile all the files even if only one has changed, which adds additional build time.
But don’t switch just yet, the article – and this I didn’t know – also mentions that piping is on the Grunt roadmap.