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Make your own ACK, because SignalR doesn't guarantee message delivery

"SignalR doesn't guarantee message delivery. Since SignalR doesn't block when you call client methods, you can invoke client methods very quickly as you've discovered. Unfortunately, the client might not always be ready to receive messages immediately once you send them, so SignalR has to buffer messages.
Generally speaking, SignalR will buffer up to 1000 messages per client. Once the client falls behind by over 1000 messages, it will start missing messages. This DefaultMessageBufferSize of 1000 can be increased, but this will increase SignalR's memory usage and it still won't guarantee message delivery.
If you want to guarantee message delivery, you will have to ACK them yourself. You can, as you suggested, only send a message after the previous message has been acknowledged. You can also ACK multiple messages at a time if waiting for an ACK for each mess…