This is an interesting and adventurous use of the filtered item list. I like it! :-)

I think the 503 suggests that the system was struggling with that big list - especially when it isn't filtered/paginated.

I would do a couple of things in the Filtered Item List settings to speed that page up:

1. Turn on pagination and limit it to 20 or so results per page. e.g., this is how this page works: https://www.wbg-localdata.org.uk/resource-hub
2. Choose a smaller image size for the thumbnails. Currently they're set big which will make for a big data transfer to render the page - especially when there are so many items on each page!

With those you should find that the page runs nice and fast!

Thanks

Joe