I don't think it makes any significant impact on the discussion list for either regular visitors or admins. Of course performance will be worse the more discussions you have in total, but I doubt you would really notice it until you get in the 10s or 100s of thousands of discussions.
Where I personally notice a performance impact is in the flags dropdown, because obviously the more flags there are the worse it will be, and it's maybe something that's a bit less optimized since regular visitors never interact with it. Unapproved content generally has a flag associated with it until you approve or delete the post.
While it cannot delete all in a single click, you might find https://discuss.flarum.org/d/30122-mass-actions useful. You can search for is:unapproved to only see unapproved discussions and then use the extension to select and perform actions on the search results. Hint: it will only select discussions that are visible in the page, but you can load "more" multiple times at the bottom of the list to increase the number of items that will be selected.
Since you have so many and if you are certain you don't want to review them, you could also delete the discussions and flags directly using MySQL commands, but I'm not sure we have any example for that, be careful not to break relationships.
