I worked on a product inside Google which used protos (v1) as the data format to a web front end, and in practice, that system was a failure, in part to the decision to use protos. The deserialization cost of protocol buffers is too high if you're doing complex data throughput, and even though the data size is smaller, it's better to send larger gzipped JSON (which will be decompressed in native code) and deserialized into JS (also via native code). We weren't using ProtoBuf.js, but our own internal javascript implementation of a similar library, and doing all of this in JS was too expensive. Granted, we were sending around protos that had multi megabyte payloads at times.

We rewrote our app eventually to send protos in JSON format to the app, while just letting our backends still pass around native protos, it worked a lot better.