Checked to see how much free space there is on the startup disk lately?
Right or control click the MacintoshHD icon. Click Get Info. In the Get Info window you will see Capacity and Available. Make sure there's a minimum of 15% free disk space.
If disk space is not the issue, booting in Safe Mode deletes system caches that may help.
A Safe Mode boot takes longer then a normal boot so be patient.
Once you see the Desktop, click the Apple menu icon top left corner of the screen.
From the drop down menu click Restart.
See if that makes a difference ...
Open Activity Monitor>System Memory (in Utilities) and check to see how much free memory there is. Also, what is it saying for Page outs and Swap used? If Page outs are more than 10% of Page ins, it means you need more RAM.
Go to CPU and see how much idle CPU there is and if there is any one program or process hogging the CPU.
Permissions repair in Disk Utility will temporarily free up some memory, if that is the issue right now.