It was a “Magewell USB Capture SDI USB 3.0 HD Video Capture Dongle Model XI100DUSB SDI”.
I did try the Magewell utility but didn’t find anything obvious to change there. Because OBS can receive 60fps without dropping frames (after a delay that’s so long it’s unusable) I think the problem is a buffering/handshaking issue on the USB/VDMX side.