In the RAC system, the source capture process becomes slower when there are no activities on one node.
Capture slow on a RAC system is associated with one node having no activities. In RAC, the capture process needs to check and makes sure that the data is in order of SCN from each node. Therefore when there is an inactive node, the pause happens more frequently.
Add a demo table into replication and have a cron job to update this table on the node with no activity every minute or so.
Balance the application load to all the nodes .
Start shareplex on the non active node as reader updates the marker table constantly so there will be some activities generated by Shareplex itself.