It's difficult to understand why a different number of parameters does change the performance
I came to this idea when I read in the mentioned sap note that internal tables passed as parameters are converted to xml and we can gain some performance if we pass them as TABLES parameters instead IMPORTING/EXPORTING (using table types).
So I thought that the number and type of the parameters could influence that time between each call, maybe I am wrong.
When I added 15 more tables as parameters that time was increased. That is why I thought the performance was related to the parameters.
And I'm not sure to understand what is the relationship with your first question which was about 2 different systems.
The issue is happening in one system and I had checked all the system parameters related to the parallel processing, it seems to be all ok.
Then I created a simple program in a different system to simulate the parallel processing and check the behaviour.
In my 'test program' all the RFCs (dummy FM, without any code, only parameters) in the loop were called in the same second.
I decided to include 15 more tables as parameters in my 'test program' and RFC, then the time was increased from 1 to 3 seconds between each call.
I checked the RFC calls in tcode STAD and it seems to be OK, the column Wait RFC is empty for all RFC calls.