low fps while ingame normal fps while tabbed out

I was having a look at the current snapshots when i noticed my fps was way lower than it was before. This doesnt have to be weird in itself because they are snapshots. but the weird part is that the fps shows normal numbers when i alt-tab out of the java screen.
so basicly:
i had (locked) 100fps in 1.12.2
in 17w48a its <60 when normal roaming around. same settings
when alt tabbed its 100 fps again.

Im not sure what would cause this but maybe something to look into for the snapshots.