This issue affects a multitude of versions, though it seems it started back in the 1.7 update, depending on the used memory there is sometimes a serious lag when taking a screenshot, the more memory being used, the longer the delay or "freeze" of the screen. The screen will literally freeze for a few seconds while taking the screenshot.

It is not always based on how much memory is being used by the game, as there are sometimes where it just takes a while to probably "save" the screenshot, and the action of Pressing F2 causes a short lag spike, as the FPS drops to 0, the screen is "frozen"

This is a big pain in the arse, if on a server, possibly in PvP and you try to take a screenshot of something that is going on in chat, you then freeze when you take that screenshot but other beings, players, mobs etc can still attack you.

Hope this is not only me.