depending on the complexity of the comp it can be a real pain.
Because when you selecty them all and then stretch them, their startpoints don’t shift to the correct time.
There might be a script on aenhancers.com
But here are some work-arounds that might work for you:
pre-compose them and then time-stretch the pre-comp.
Or
render to a higher framerate (eg. 35fps), re-import and comform to your original framerate (25 for PAL) this would slow it down to 35/25 = 140%.