After some research, I think I found a solution.
Navigate to Network → Global Configuration → Host Name Database
In this text field, add your custom hostnames, one by one, in this format:
127.0.0.1 localhost truenas.local
127.0.0.1 localhost truenas.home
127.0.0.1 localhost guestnas.home
…and so on.
A server restart was required.
Now, connecting or reconnecting to an SMB share happens almost instantly. It doesn’t matter which hostname is used. It doesn’t matter if it’s from Windows or Linux.
I have tested this multiple times, over and over again, and it appears to be a consistent fix.
No longer does Windows File Explorer hang for over 30 seconds just to connect.
I don’t believe this is unique to TrueNAS Core or FreeBSD. I think it’s due to a new version of Samba that was introduced some time between the release of Core 13.0 and 13.3.
In fact, this likely also applies to newer versions of TrueNAS SCALE.