No, it doesn't. What you can do is use an RSS proxy to create your own feeds of any site and read that through Nextcloud News. I do this for a few specific sites and purposes - e.g. custom searches on auction sites.
As to Nextcloud News being slow with 50K unread items I wonder whether there's something else amiss as I have close to 600K unread items without undue slowdowns.
The only real slowdown I notice in Nextcloud News is caused by a dog-slow piece of javascript in news/templates/part.content.php which causes a delay any time an item is clicked in the feed. This can be solved by removing everything related to 'sharing' (i.o.w. everything after </time> in the list div (class=utils), make sure to balance those tags...). News is much faster in use that way and since I never use the 'sharing' related stuff - if anything needs to be shared I can just share a link instead - nothing of value is lost.
