15s wait for prompt for sudo su - on -current

From: tech-lists <tech-lists_at_zyxst.net>
Date: Sun, 13 Jun 2021 14:21:02 UTC

context is main-n247177-dab84426a68
# uname -KU
1400018 1400018
# uname -mn
rpi4 arm64

disk is usb3-connected spinning rust. OS is on microsd

I'm seeing, recently, a roughly 15s wait for the prompt to return after
sudo su -

sudo is aliased to doas

The problem does not happen on stable/13-n245218-41d864457dd on the same
hardware. DNS works fine both for itself and externally. How can I
fix/debug further?