I’ve measured the speed improvements again on beta 13, this time on the todos app. Here are the results (in all cases I did all actions twice and recorded 2nd result):
Building / cold start
Meteor 1.4.1: 40 seconds
Meteor 1.4.2: 24 seconds
Server code change - rebuilding
Meteor 1.4.1: 3.5 seconds
Meteor 1.4.2: 1.8 seconds
Client code change - rebuilding
Meteor 1.4.1: 8.5 seconds
Meteor 1.4.2: 1.7 second (5x improvement!)
Overall, I like what I see 
PS. What I would like even more is speeding up meteor build time, especially (but not limited to) when iOS and Android are added as platforms.