"..the request to ISIM are coming one by one (gap of 1 sec) which makes sense as it iterates.." Nope. It does not make sense.
A solid 1 second gap is very rare. The only possibility that this condition could happen is the fact that you have system.sleep(1); command in your AL, which basically delay the TDI process... by design. People do add a sleep command if they know that the input (data source) is transactional type of data source, where there is a possible two updates (e.g. one update phone, the other update the address) of the same user on the same iteration. So, adding sleep command will reduce the chance to have a race condition within the ISIM system.
Technically, yes, you can find many ways to do multithreading on TDI system, but based on my experience, people tend to slow down on TDI processing, rather than speeding it up.
Having said, if this is found to be ISIM issue, then follow what Mr. Franz said. The statement that makes me wonder is the solid 1 second gap, as a not fine-tuned ISIM would generate a more disperse outcome rather than a solid 1 second gap.
just food for thought.