Hi @Saad, see Approaches to stress testing ODK servers to try to tease out where the bottlenecks are in your config.
The thing that comes to mind is that Aggregate splits submissions into individual values that are insert into the DB, so my guess is that the bottleneck might be at the DB layer.
Central doesn't do this sort of submission splitting and so it can be a lot faster. Have you tried a similar deployment with Central? See https://docs.opendatakit.org/central-intro/#notes-on-performance. I'd be interested to see how it performs for you.