Canonical tags set up correctly? Basics in place like meta data? Content rendering in the HTML? Google will generally index a page pretty quickly, even from a site with lower authority.
In the Google Search console you can usually see the reason why it's not included. Page speed is rarely a reason for indexing / non-indexing unless it's really bad.
I've clicked into my non-indexed pages, and everywhere that people say it should say the reason why the page is not indexed, there simply isn't a reason mentioned. It just says the generic crawled but not indexed even when clicked into specific pages.
I've even watched YouTube videos of people going into their Search Console dashboard to make sure I'm not missing anything (and indeed I've seen where those people for some pages do see a reason, but for mine I do not).
I don’t think it says anything useful for crawled but not indexed, unfortunately. My suspicion is that it’s almost always backlinks, but not totally sure.
