Why waste the time of having the ref come over and look at the play on a tablet? Ther should be an official eye in the sky at every game with immediate access to all angles correcting clear and obvious errors. 95% of challenges would go away, as would the tedious delay to review them on the sideline.
The review of completions / incompletions, fumbles, spots and TD's could be happening during the 40 second play clock. On clearly disputed play calls, the ref on field could momentarily pause for confirmation if need be.
This wouldn't resolve penalty disputes, but would speed up and correct obvious errors that result in long delayed challenges or teams getting hose by bad calls.