Not enough free space on the disk can slow the system down.
Click your Apple menu top left in your screen. From the drop down menu click About This Mac > More Info > Storage
Make sure there's at least 15% free disk space.
Try removing login items. Open System Preferences > Users & Groups then select the Login Items tab. Remove all items.
And go to HD > Library > StartupItems. Remove all items from the StrartupItems folder.
Check the startup disk.
Launch Disk Utility located in HD > Applications > Utilities
Select the startup disk on the left then select the First Aid tab.
Click: Verify Disk (not Verify Disk Permissions)
If the disk needs repairing, restart your Mac while holding down the Command + R keys. From there you can access the built in utilities in OS X Recovery to repair the startup disk.
Be aware, OS X Recovery requires a high speed broadband connection.
And backup all your important files before repairing or restoring OS X just in case.