Convert Array of Client to Object of Client

Hi,
I have an issue about find specific client for sending message. Now this.clients return an array of objects and think about 500 players seperate to 100 group need to private chat each other.
Fastest Solution for this ChatBox is save Client Object to each Player and use this to send.
But I think this is the course of Client leave room but the Web Socket not disconnect in https://github.com/colyseus/colyseus/issues/361
If I loop in the array of 500 client to find correct clients ID and send each message (1s/mess), this will be a nightmare for game performance.
It would be easier if Room can get specific client with id and send it with this.clients[id].send