I haven't been on many delayed flights where I'm in F, but the last one I can remember (SEA-YYC), there was a PDB service.

I think slow domestic seating is a combination of the checked bags issue, but also because DL (and most domestic US carriers) use boarding processes that reward status & spending rather than time/speed efficiency, as a way to encourage people to pay/fly more for better boarding. Faster boarding procedures include random boarding, or a window/middle/aisle method (which could be combined with back to front). These mean that people are less held up by someone stowing bags, and that there's less waiting for the aisle person to get up to let people seated in the window/middle seats into the row. Whereas Delta's main cabin procedure of C+ > Sky > 1/2/3 > Basic means that you're generally boarding from front to back (such that someone stowing bags holds up the line), and the aisle & window seats will generally fill first (meaning more time for people to get out of their seat to let the middle people in).