I was under the assumption to interact with signed content and while I see a performance challenge, not checking the signatures at all cannot be the solution. I'd be fine with a background thread checking signatures, especially of not explicitly trusted relays. So if I explicitly trust relay.x to check signatures, the client could check events from relay.y for either having come from relay.x, too or in a deferred manner check signatures and if any invalid signature is found, flag the relay as bad mark events from it as not checked until they are checked.