One commonly reported problem with Yosemite comes from Apple Mail users, who have external keyboards (USB or Bluetooth), where typing in Mail can sporadically become delayed and slow. This only happens when using an external keyboard, and only seems to affect the Mail app.

Rumor has it that the forthcoming OS X 10.10.3 update fixes this issue. Two workarounds that have helped some are changing the default font in Mail (Helvetica Neue seems to work), or turning on Slow Keys in the Accessibility System Preference pane with the delay set to the shortest setting.  https://discussions.apple.com/message/27736539