Mongo BI Connector and Mongo DB slowdowns

We are using tableau integration with mongo database and mongosql converter since there is no native support for mongo db. When hitting collections with ~2million records, we are experiencing slowdowns. Since the queries from tableau can be very dynamic, it is very hard to set appropriate indexes.

I am not yet sure if slowdowns occur on mongo level or on mongosqld converter level. Currently, the converter is pretty CPU-intensive, suggesting it may have bigger impact on slowdowns than query speed.

What do you suggest in this case?
What is your experience with mongo sql converter and tableau?

Thank you,