Driving me crazy. My iPhone gets drunk on unadulterated wifi at my desk, while my 5k iMac downloads in tiny little sips.
It's a well known issue with Yosemite. The source of the problem is believed to be
discoveryd, which is the new DNS and network discovery service that replaced mDNSResponder from previous versions. They replaced it in order to support stuff like Handoff, but it's extremely buggy and likes to do really weird things to your home network.
I wouldn't recommend this solution, especially with El Capitan around the corner, but this article explains it a bit more.
EDIT: this is another fun article that dives even deeper into
Tried this one, and it's working great. Had to kill AirDrop and ADML, though. http://www.engadget.com/2014/11/25/heres-the-root-cause-and-a-workaround-for-ios-8-and-yosemite-wi/
Yup, it's absolutely terrible.