> It's much better ux when a user downloads the whole catalogue and then apply filters on the client without having to touch the server until he wants to get to the checkout.

This is what we[0] do too. We have a single JSON with a thousand over BOMs that's loaded directly into the browser. Previously we loaded the inventory data via an API as is usually expected. The fact that there's even an API meant requiring progress and loading bars, API unavailability scenarios, etc.

Having it all as a single preloaded JSON meant that all of the above goes away. Response is instantaneous.

[0]: https://chubic.com