If Photoshop is slow when it opens, working in the background
Photoshop sometimes needs a lot of time to start, even if only to communicate with the graphics card; however you may face this strange situation, when Photoshop opens fine and fast, yet the moment you try, for instance, to open a file, going to the File > Recent menu, Photoshop steals focus and your menu disappears, forcing you to reopen it!
At this point you have two choices. The first one is to wait a few seconds after Photoshop opens, to let it finish what it does. The second choice, if you prefer a snappier response, is to discover what plugins or panels cause this delay.
In my case the problem came from the Libraries panel. When I closed it (even as an icon), this delay disappeared and I could start working in Photoshop instantly.
In any case, even without this delay, I prefer a lighter program such as PhotoFiltre, for extremely simple tasks, such as pasting a clipboard image to resize it and use it in a mail message, etc. (Photoshop won’t even let you paste an image immediately, you have first to create an empty one in the clipboard’s dimensions! Wow! That’s very intuitive!)