So that leaves the bottleneck on either the merchant bank, the US card networks, the payment terminal or the retailers internet connection. Still a crapshoot to figure out where the latency comes from, but at least it's not the cards or the issuing banks.
My assumption is that it's the payment terminals. The US tends to have far more dated/low end terminals than I've seen elsewhere. I have a feeling that since retailers are forced to upgrade, they choose the cheapest, most terrible option they can.
I'm not so sure most of the fault lies with the terminals. In April 2016, Visa announced a software/firmware update to card terminals to speed up the awkward waits affecting chip cards [1].
But to your point I found chip terminal manufactuer that claims to have optimized the communications between the chip and the terminal [2].
