Unplayable frame rate without v-sync

My DE is XFCE and I have Intel graphics. Games such as Minecraft run at <5 FPS when full screen and in game v-sync is turned off. When v-sync is on, it runs at 60 FPS. What is causing this? This happens whether the compositor is on or not, the only difference being screen tearing when the compositor is off.

When played in windowed mode I get ~10 FPS independent of whether v-sync is turned on or not. Does v-sync only have an effect in full screen mode?