The sizer has a slot for carryons and a slot for personal items. In my experience they don't always make everyone size the personal items if they're obviously small enough to fit under the seat. OTOH, one gate attendant told me she had to bc if she didn't and her supervisor saw her not doing it she'd get in trouble. I don't know that it has ever delayed a flight I've been on - they usually try to do it before boarding starts.

On Concourse A (where all Frontier flights are) it would take maybe 10 minutes at a normal pace to get from one end to the other. I don't know whether they routinely hold planes but they are usually aware of tight connections and may request that people let those in a hurry off first.