Fixed Frame Rate problem / fps drop

any levels run fine at 60 fps
but as soon as i try to use fixed frame rate
to limit it to 24 fps, (i need 24 for cinematics), it drops to 9-10 fps.

happens either when i check this “use fixed frame rate” box,
or type in the console command t.MaxFPS 24

the thing i don’t understand is, it can handle 60 fps just fine. why ?

pullin my hair out on this one, cause i never used to have this problem. maybe i’m missing something obvious, hopefully someone knows.

this is on any level, default, new project
both in 4.19
and newly installed 4.20

on mac osx high sierra

I got it working, by reseting the PRAM.

although, i don’t understand why that would work.
hopefully it lasts.