List of steps to reproduce (step by step, including full links if applicable):

• Create a large number of namespaces (~500).
• Visit either Special:Watchlist or Special:RecentChanges

What happens?:
The browser becomes very unresponsive and the enhanced filters can take up to a minute to fully load. Interacting with the filters also becomes tediously slow.

This happens because of the extreme complexity of the DOM with OOUI and such a large number of namespaces. It takes the browser a long time to generate and render the resulting elements.

What should have happened instead?:
The web page is responsive and the filters load in a reasonable amount of time. This can possibly be achieved by lazy-loading the namespace filters.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:
MediaWiki 1.35.6 with enhanced filters enabled.