The "zippy" client stuff comes at a cost: my browser has to download ~150kb of javascript to make this website work. Not as bloated as many sites but more JS than is truly needed for this sort of thing. Likely framework bloat.

Edit: I should say of course you can have zippy client-side stuff without the bloat (for example: just filter the JSON object and update the DOM yourself without a framework!). So it doesn't _really_ come at a cost, just typically the most expedient way a reactive UI is put together uses a framework like React.