Not a performance concern, but I notice that the settings page is a navigation dead end. Once you get to it, there is no link back to the Special:GlobalWatchlist page, you're forced to use user history. And once you save changes, there's no link to either Special:GlobalWatchlist or Special:GlobalWatchlistSettings in its default state.
Refreshing the page at that stage doesn't work, you get stuck with seeing this:
I think that the experience of the settings page should match Special:Preferences. After saving your changes, you should still see the form. And there needs to be a button to go back to Special:GlobalWatchlist.
You'd need to consult a designer from staff on this, this is not my speciality. There is a consistency issue here, we can't have every extension have a custom settings page that behaves completely differently than the wiki's or other extensions.
If the preferences page is extensible (via a hook, maybe?), I think it would make a lot more sense to have these settings added to the "Watchlist" tab there rather than having a dedicated new page.
In the current user script implementation, there is a button at the top to go back, and when you save you are automatically redirected to the global watchlist
I understand not wanting to deviate from other preferences forms though.
I know @IKhitron has been using the user script and has already tested the extension on beta - what do you think about how to navigate back?
It could be added to the normal preferences, but I couldn't figure out how to give the need for users to be able to add new rows for sites (and I think some other issues I ran into) so we opted for a dedicated special page.
I can only test so many functionality, because I have no multiple watched pages on beta wikis, of course. I did find many problems and intended to talk with you about them this evening, but as I can see, you've filed tasks for all of them today. For your question, I expected to see autoredirect to Special:GWL as in the script, and was surprised there wasn't. If it was my call, I would have four buttons at the bottom of the settings page: "Save", "Save and go back to the GWL", "Clear", "Clear and go back to the GWL".