The worst boarding experience I've ever had was on a 75X boarding L1. It was an early morning flight SLC-ATL last December, and the sun wasn't even up yet. For whatever reason, the inbound flight had parked at one of the end gates in the D concourse (D13 or something like that). Normally 737s, A320s, and CR9s park there. Boarding F wasn't the issue. It was boarding the other 160 passengers. Many factors came into play: the galley area by L1 is tiny in comparison to the big open bulkhead area by L2; the F cabin on the 75X has 26 seats, and is only superseded by the domestic 767 cabin (but at least they have two aisles, not one). So when FAs were trying to take drink orders, take coats, and serve drinks, all while 160 passengers try to squeeze down a single aisle with overstuffed backpacks, coats, and luggage that looked like it was bigger than the 60 linear inch max allowed for checked bags, it quickly became chaotic.

That was the only time I've boarded a 757 and not used L2.