Why is IntelliJ IDEA hanging on “Indexing”?

Answer 1

In PhpStorm, what solved this for me was excluding folders I didn't need to be indexed from the indexing (specifically the vendor folder, a caches folder, and a few asset folders that contained thousands of images). Instantly it began making progress and completed.

To do this:

1. in the project directory list, right click the folder you want to exclude
2. Mark Directory As
3. Excluded

Answer 2

Try Invalidating the cache and restarting IntelliJ.

In the File menu, select Invalidate Caches / Restart... and then click the Invalidate and Restart button.