About 4 seconds.  Network and firefox are working fine.  No, I didn't zap the settings.  That's a total PITA and one of the things I dislike most about this device is it sometimes does that itself on a firmware upgrade.  That fact alone makes it difficult to recommend it to people, even though I like the radio in it a lot, and it does Guest SSID (add VLAN tagging and it would totally rock).
No, I don't run a software firewall here.  We're behind a Sophos UTM/Astaro.
No, I am not going to turn off all devices while testing.  That's another completely useless, household-disrupting PITA.  I'm gigabit wired to it anyway so it's not the wireless.
I'll try a reset/config restore though.
Actually, looking at it further, it's only over https, no delay over http. Wonder if OP was using https too.