Was a worthwhile change. Also updated help-steps/chroot-raw to this syntax.
But it didn’t fix the issue. It’s probably actually doing the very same thing as the old syntax.
That’s what help-steps/chroot-raw does not do. No mounting of sysfs. Therefore that is probably why previous calls to update-grub (by various Debian maintenance scripts by various packages) do not result in such major slow down.
grub-install needs sysfs. It fails without it. Tested.
But update-grub does not need sysfs. Tested.
Perhaps we can unmount sysfs after grub-install but before running update-grub? Tested. Build succeeded. Didn’t test boot.