Hi Thomas,
Could you please indicate the queue where you see the thread assigned/working = 10/10.
a full screenshot would be useful.
this would help to understand if the bottleneck is from sender adapter to integration engine or from integration engine to the receiver adapter.
it may be your receiver adapter taking time to process the message and in result other message are kept waiting in the queue.
you can also try to put timeout as in the answer given by Sreenivas