how you normally would - +

https://community.bistudio.com/wiki/setAccTime

you can use the console when testing an editor mission to use this

@b0st0n bring up the Gcam UI then in the bottom right side there is a box with the units around you just near that box you should see a + & - by pressing theas buttons you can speed the world up to 4x speed or you can slow it down to 0.1 or compleatly freze the world.
Sorry for late Reply hope this helps if you haven't sorted it out already, Cheers Jez

Alternatively, recording at 60fps and playing back at the cinema standard of 24fps gives you slow-mo. No need to do it in game necessarily.