NetBSD 10.x turns off the display when booting off USB 3 SSD

[lucho]

The NetBSD 10.1 kernel turns off the display when booting off a USB 3 SSD on my ProDesk 600 G1. (The 9.4 kernel doesn't do that, nor does 10.1 do it if it boots off a CD.) When booting the 10.1 kernel with the -a switch, after it asks for boot and dump devices, and I enter the kernel debugger (Ctrl-Alt-Esc) at the last question about the file system type and issue the "continue" command there, it starts outputting repeatedly

umass0: BBB reset failed, TIMEOUT
xhci0: xhci_set_dequeue: endpoint 0x0, timed out

Also, both 10.1 and 9.4 do a weird thing on that machine: a few minutes (how many? it's different each time!) past bootstrap the "Dhrystone" and "Coremark" integer (but not floating point) performance drops about 5 to 7 times while idle state CPU load remains near zero and CPU temperature normal. This problem doesn't exist with the other 6 operating systems on that machine nor with NetBSD 10.1 on another machine.

I've attached the "/var/run/dmesg.boot" file of NetBSD 9.4 (can't do that for 10.1 because it doesn't go up to the point of mounting the root device).