semi related question, if a client is poorly built and thus really bad at managing relay subscriptions (ie they do a ton of different queries on every subscription on every page load) is that costly for relays?

That's a question about the protocol in general being based on JSON riding HTTP vs something more compact, binary on the wire.

Clearly tradeoffs were made favoring adoptability and low barrier to entry over performance and scalability.

good point !

potentially reactions could exist for a certain amount of time (time to live or ephemeral) so this data does not become so costly

reactions in lens are purely centralized :) .. it seems in Lens are not worth Matic and Gas and they decided to keep it centralized

No and I've been mulling over a free relay policy that expires them earlier than anything else. Either that or rejecting them entirely as spam. On one hand, likes are a debatably toxic form of human engagement, on the other hand, relays are supposed to be dumb and let the end user devices figure out what to do.

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we can do it on SSR, any idea about that?

Without reactions it often feels like screaming in an empty room.

Yeah counting them on the client side is absurdly expensive. But reactions are super valuable in my book!