This is the most efficient way I can come up with. We need to query the Site model as that is where site root pages are configured and there's no other way to reliably tell if a particular page is a site root.

I've pasted the SQL (postgres) that this compiles to below. As you can see, its only asking the database if a record exists and is not concerned about the data in the record or how many records there are, so I'd expect this query would run pretty fast.

```SELECT (1) AS "a" FROM "wagtailcore_site" WHERE "wagtailcore_site"."root_page_id" = 2 LIMIT 1```