so if a user has subscribed for that kind of event that will make the event to immediately go to user. the relay has to collect before user sees it, which slows down the event propagation. 

alternative way could be to encrypt for the relay operator? then relay can credit the pubkey that generates the keychat event beforehand and accept the following events? 

in your design does each relay get a different 'cashu stamp'?  

 Yeah, Cashu supports Pay to Public Key (P2PK). 

ehttps://github.com/cashubtc/nuts/blob/main/11.md 

Keychat stamp is just normal cashu ecash.