I have a recently setup Fios gigbit install with the Verizon-provided Router/Wifi.  I've connected my AC1200 S4's to this router.  The S4 closest to the Verizon router is connected via the TP-link provided Ethernet cable.

Positioned 6 feet from both routers (they're within a foot of each other), if I connect to the mesh wifi network I've defined in my TP-link, when I try to watch a youtube live stream, it will typically hang (shows the youtube spinning circle in the video frame) after 5 to 15 seconds.

If I take my same laptop (Mac) and connect to the wifi through my Verizon router, the exact same youtube stream will stream without hanging.  I can repeatedly switch back and forth between the wifi networks and the youtube stream consistenly freezes when connected through the TP-link wifi, but consistenly continues to stream when connected through the Verizon wifi.

How can I debug this?  If this can't be eliminated, then I'll have to return the TP-link and try another mesh solution.