Why is my Mac slow after the OS X 10.10.3 update?
Considering the limited information you provided, here are some generic steps.


Open the Activity Monitor (located in your Applications/Utilities Folder).


Sort by CPU and look who is the most active app in there. 
Also take a look at your memory and your Disk activity.

Next open your Console log (also in your Applications/Utilities Folder) and look for repeating message.


Those two steps should give you some idea what is slowing it down.