Yep, for off sides the video ref should only look at the replay once and if it is inconclusive to the human eye it's onside.
If it has to be played 2 or 3 times in slow motion from different angles it means it is not clear and obvious as you say.
The job is too tough for a linesman because they have to keep one eye on the ball from the precise moment it's passed or crossed, and the other eye watching the line from the precise moment the ball leaves the foot, and also try to spot any off the ball incidents in free kicks.
What would make...