tightly-coupled memory performance !!

Actually I just mean look at the "calling function assembly code" to see if some of the instructions before the 'call' are omitted for the second one.

Assuming all the instructions and data in the called function are all located in the tightly coupled memory, and if the code executes the same instructions between calls then it should have the same execution time. But that doesn't mean the stack operations leading into the call to the function will be the same between two calls. So you should look at the instructions before the 'call' to make sure you are simply not seeing additional work being done for the first call that is omitted for the second one.