Profile your consumer and see what takes time.

Everything is okay, just that the logic contains a REST call that takes around 15-30 seconds in worst case. I'm thinking to make system resilient when majority of messages are having REST call that takes around 15-30 seconds in worst case by introducing Consumer Thread Pool or by any other way possible.

• Only 1 way I found till now to improve consumption rate is via "increasing Number of Consumer". Does high number of consumers have any drawbacks/concerns?
• Will try to play around with Prefetch Settings and Size of Consumer Thread Pool? Is it correct to say, Prefect Count should be greater than Consumer Thread Pool?

How would 15 threads parallelize work over one message?

I was thinking of a case where internally library might using Divide and Conquer Approach (or something similar) to optimize a particular flow.

Only 1 way I found till now to improve consumption rate is via "increasing Number of Consumer". Does high number of consumers have any drawbacks/concerns?

It's hard to say, but you can use a few dozens in a given connection without problems.

You can start experimenting with a few channels (10, 20, etc) and one consumer per channel. Considering the message processing is long compared to the actual delivery, set Qos (prefetch count) to 2, this way you should have a message ready when you're done processing. Set the size of the thread pool to the number of channels.

There seem to be relation betweenMaximum Concurrency andThread Pool Size, which were not set properly as a pair. Doing this makes the Thread Pool working as per expectation. Got to know about it via Hit and Trial on my configs. Not found it on documentation. Wrote an answer on Stackoverflow regarding the same:https://stackoverflow.com/a/72869716/5662835
for others dev folks.

Thanks for the follow-up and the answer on SO, users will appreciate it for sure.
You can also have a look at theprefetchCount (QoS) setting for fine tuning. Default is 250 but you may want to use a smaller value, as suggested in the note.

Yes, Prefetch count can be configured also, and should be low in scenario where processing of Single Message is time consuming coz in that case 250 (by default), 249 will be blocked and will get a change late and new message can be proceeded before them in some other consumer.
I have one question or I'm not able to visualize/understand a scenario where playing around withPrefetch Count value can increase the consumption rate?