You’re likely getting time out errors due to the sheer volume of users. The WP reassignment routine isn’t very efficient. If a user’s post were already reassigned before deleting them, removal should happen in the allotted time.
You can reassign the author of a page’s worth of posts to another user through the bulk edit feature on the post lists table. You can increase the number of posts on a page through the screen options tab at upper right. I think you are limited to 99 posts per page.
If a user has so many posts that even 99 is not enough, you could use the interconnect/it script to update a particular post_author ID to another. AFAIK changing the post_author value is all that is required to reassign a post. I’ve not thoroughly investigated reassignment this way. Backup your DB before making changes like this. Make only a few limited reassignments at first and test thoroughly before making widespread changes.