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It's a mistake to use this bit for ZCL commands that don't have a response (like the on/off command). You only really need it for those requests that have a response, like a read attributes. Otherwise, you'll see what you're observing which is that the request is sent out, but after the 5th one, you have will need to wait until space clears out in the queue (which takes 15-20 seconds as observed).
The recommended setting is to turn it off for requests that don't require a response.