I see that the html code for the prosilver templates appears in files in the styles/prosilver/templates folder and separately appears in the phpbb_styles_template_data table in the db.
I gather that the reason for the duplication is so that when an admin edits a template in the ACP, if the server is set up to disallow writing, the changes can be stored in the db. Is that correct?
Does it slow up the site noticeably to be using db templates rather than file templates? Is that something an admin should try to avoid? And once the admin makes a change in the db version, is there a way to get phpbb3 to start recognizing the file version again? Is that as simple as re-saving the file version?
The difference all depends on the speed of your Database Server to the Apache server, sometimes it can be just as fast, sometimes a bit slower.
Everything will end up cached, however, so it ends up being about the same speed.
You can switch the templates from database to file system and back again at any time. But once you make another change, it will switch back to DB.
You may not retain any changes you made to the templates in the DB if your server does not allow phpBB to write back to the template files.
You can also choose to "export" your style, which saves the most recent version of all of the templates (including DB and files).
Which you can use to replace the files you have to be sure you have the most updated version.