Since the release of Photoshop version 2015.5, a number of Adobe users have complained about their systems slowing down to unusable levels or of bugs never seen before in other tools. Besides the usual forum complaints, some have even recorded videos showing their lack of performance.
Below is a list of Solutions to known issues:
I. Install Photoshop 2015.5.1 Update released (8/8/2016)
Solves a number of problems listed HERE.
II. If you have slow menus or laggy commands = Turn OFF Font PreviewType > Font Preview SizeChoose None
III. If Photomerge is slower than expected = Increase your cache levelsPhotoshop CC > Preferences > Performance.
Change the cache levels to 4, and then click OK.