how are you iterating over the keys in the DistributedCallable?
Any updates on the issue you encountered? It seems there are many moving parts and variables in your application and potential cause of the slowdown could be hidden in any one of these. If you want to isolate and focus on distributed execution I suggest that you try PiApproximationDemo from Infinispan examples. In your network setup try a few runs of PiApproximationDemo with a single node and a few runs with multiple nodes and recored/observe results.
Pedro and Vladimir,
Thank you for your replies. I am stuck with some other work at the moment and I will get to the issue within a few weeks... I will certainly try the PiApproximationDemo and will inspect other variables in depth.