Lots of nodes, many don't need to be in sitemap

Hello.

We have a large website with about 1,000,000 nodes. 95% of those don't need to be in the sitemap. Nevertheless, it seems because rebuilding / updating sitemaps is still cycling through every node, sitemap regeneration keeps failing.

I excluded the content type from the gui, and ideally, I would think that sitemap should not even look at those nodes. Nevertheless, even though our sitemap nly needs 50,000 links, we need to cycle for hours through 1,000,000 to generate the sitemap.

Are there any hooks where we can exclude a content type so its not even taken into consideration?

Thanks!