DocumentDB persistence throughput

I'm using DocumentDB with Spring services through Spring Data MongoDB and am having issues with high data throughput.

The service needs to load, update, and persist approximately 15,000 documents from a collection, each document around 5kb in size. Using a combination of Spring repositories and Mongo templates I've got the load time to around 20s and the persistence to around 40s, I'm trying to find out how or if it's possible to lower these values significantly.

The collection has 3 indexes on it, the DocDB cluster has a primary and replica instance of r6g.2xlarge size. I have Performance Insights enabled and it shows a split of 50% I/O and 30% CPU during the process.

Is the performance I'm seeing expected or is there a way to lower the time taken to load and save the data?