Server Does Not Receive Heartbeats

[zx spectrum]

A client logs in to the server, and server starts sending lots and lots of
messages back to the client. At the same time, a client needs to send
HEARTBEATs back to the server every X seconds.

login                   =>   [SERVER]  <----------  [CLIENT]
send messages   =>   [SERVER]  ---------->  [CLIENT]
send heartbeats   =>   [SERVER]  <----------  [CLIENT] ( at the same time )

Seems that heartbeat messages do not reach the server until it is done
sending all the messages. After all ( e.g. 10,000,000 ) messages are sent to
the client, the server logs that it is now receiving those heartbeats that
were sent before.

I can't really tell whether or not it is client's fault of not being able to
send the messages out (downstream) under a heavy load through the same
channel:

            clientChannel.write( ChannelBuffers.wrappedBuffer( message ) )

Or it is the server, which is not capable to accept those heartbeats until
all messages are sent out to the client.

How can see where exactly "very late" heartbeats sit? And what is causing
the channel lock in the "client --> server" direction?

Thank you!
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