For a compact message format being sent to browsers, you might look at messagepack, eg with https://github.com/msgpack/msgpack-javascript – it's essentially binary JSON, so it'd be compatible with OpenAPI specs.

I think the ser/des is slower than JSON in most browsers, but the message format is smaller.

Oftentimes, using a query parameter like `?exclude[]=…` or `?include[]=…` or similar to say "only get me these response fields, not the whole object" can be useful for this too (and then you still get JSON back).