There was some version of Stylus dependency, that did build very slowly, because it had bug, where it did spend a lot of time in loop doing nothing important. That is why I converted my Stylus code to CSS:
Also, there has been fixes to Meteor to improve build time. When building with Meteor, it is possible to see Meteor profile, what time each dependency and step takes. Some dependency taking too much time can be a bug.
METEOR_PROFILE=100 meteor build .build --directory --platforms=web.browser