I have multiple standalone NiFi instances (approx. 10) that I want to use to send data to a NiFi cluster (3 NiFi instances) using RPG (Site-to-Site). But, the flow from the standalone instances to the cluster seems to be slow.
You may want to first rule out your network. You could ssh to one of the standalone nodes and then try to SCP a large file from the standalone node to one of the nodes in the NiFi cluster. If that is slow then it is more of a network problem and there won't be much you can do to make it go faster in NiFi.
In NiFi, you can tune each side of the site-to-site config...
On the central cluster you can right-click on the remote Input Port and configure the concurrent tasks which defaults to 1. This is the number of threads that can concurrently process data received on the port.
On the standalone NiFi instances you can also configure the concurrent tasks used to send data to a given port. Right-click on the RPG and select "Manage remote ports", and then change the concurrent tasks for whichever port.