One commonly reported problem plaguing a percentage of Yosemite users is extremely slow WiFi performance, and/or frequent WiFi connection drops. One Apple Discussion Group thread regarding this issue has now grown to over 138 pages, with over a half million views: https://discussions.apple.com/message/27382058
This issue isn’t limited to a specific Mac make/model, and has been reported to occur with a variety of WiFi setups including brand new AirPort Extremes.
It was hoped that the OS X 10.10.1 update would resolve this, which reports to “improve WiFi reliability”, but it has not. Affected users testing the forthcoming OS X 10.10.2 beta also are still experiencing the problem.
One workaround that has helped some experiencing this Yosemite bug is to disable Bluetooth. The only other workaround is to use Ethernet instead of WiFi, until Apple fixes it.