Hi Gautam
Can you clarify if you are using RFC or proxy call to the ECC system?
I'd suggest that you check the timestamp in the audit log of each call. Note down the start and end times of each of the parallel calls at the point when it calls the receiver channel.
If the start time of each call is near one another, then it would most likely mean that the receiver channels are acting in parallel and the delay might be due to the ECC system processing.
If the start time of each call is far apart, i.e. start of second call is after end of first call, then this might indicate queuing on PI side and only 1 call to the receiver channel is being executed at a time.
Once you have a rough idea where the bottleneck is, then you can further zoom in to it.
If the delay is in the ECC system, you will need to get the help of an ABAP colleague to perform a runtime analysis/trace on it. Maybe certain tables are locked during processing, so it only allows sequential processing and not parallel.
Try this out and if you need further help, please provide some more info based on the troubleshooting steps above.
Rgds
Eng Swee