Pentium 1 @ 75MHz is quite old, sure. But unless you have a specific need, there's little reason NOT to use the newest version of NetBSD. It's not like we've dropped support for old PCs or anything like that! That system would benefit from a bit of tuning, though -- disabling lots of services at startup, possibly a non-GENERIC kernel, etc.
--- (This is an aside, but this not the first time I've met someone who feels they need to use an older version of NetBSD to "match" their older hardware. It's strange to me, because I thought people are turning to NetBSD in the first place because they know that even our very newest code supports their older platforms.)
The first boot is always the slowest, no matter what port you're running, because we generate some crypto keys, etc. You might have already noticed that Raspberry Pi images reboot themselves in the middle of the first boot to resize the filesystem to match the SD card's capacity. (I'm assuming it's the same on the RPi Zero).
I'm not sure what it means for the password: prompt to "time out," though.