I've been talking a bit with a_thing about this in #nuxified IRC channel. I find yum to be quite slow. Not only does it apparently update the package database every time it is ran (which even on slower broadband networks can annoyingly slow down the overall install process), but calculating dependancies takes a bit longer than I'd like.
Many of us probably remember the old "dependancy hell" often associated with RPM based distros due to their poor management of dependancies. It was happening when you download an individual RPM package that had alot of other dependancies, and by downloading it you didn't know which other packs to download inevitably ending up running the RPM file numerous times every time getting a message saying that this or that dependancy is missing and then going to search and download it.