I see the same behavior in large chain stores such as Target, Lowes, and Walmart that would have dedicated private lines between the stores, their datacenter(s) and the banks. I've worked at a few large retailers and they typically had a hub and spoke model of network where all the stores would communicate over a private MPLS network to the datacenter where we had a "payment switch" (also called a payment gateway or router) dedicated individual redundant T3 lines to each card network (Amex, Discover, Mastercard/Visa, EBT, and gift card processors). This was at a chain with 200 stores. Larger chains would be even more robust I would imagine.

I think the delays aren't network related, but more along the lines of the process that is happening. With mag stripe the approval flow was much simpler and happened in fractions of a second end to end. EMV is a different ballgame unfortunately.