I agree it merges early and you can never tell how long it will be cause if they have all the switchbacks open on the platform (which you can't see whether they are or not at the merge point) then it takes much longer.
If the switchbacks aren't open and you time it just right you can be on the train in about 10 minutes from the merge point.
If the switchbacks are open, and if they have multiple
scooter/stroller/wheelchair parties coming up the elevator that they are letting in front of you repeatedly while you are going up the steps, it can be a good half hour or so from the merge point.
When the lines are long I've also stood at the merge point for quite awhile while the worker lets the main line go through ahead of us. I've had several times in Kings Cross where EP has not saved us any time at all, and even a few instances where it took us longer cause there were people walking through the merge point while were scanning and we would have just been better off walking the regular que if we'd known.
If you are in a hurry- walking is definitely better!