Your Mac should have plenty of power to run Yosemite without lag. Maybe you have a utility or something running that is conflicting with Yosemite.
Hold the shift key when booting to boot to safe mode. That will bypass any login or startup items and eliminate that as the cause. If it works well in safe mode, that tells you there is a startup or login item you need to remove.
Part of it is Yosemite is reindexing everything. There may be some third party app's that need updating. Personally, I opted for a clean install (auto reflex left over from WinDoze). No lag on my late 2013 rMBP.