Performance problems with multiple database connections?

I have a team that is working on DreamFactory Services that connect to three different databases through a connection string and stored procedure. They are reporting that when they introduce multiple databases, the services are having performance issues. (These are connecting to MS SQL.)

Has anyone seen this type of issue?

What interface are they using when the performance issue occurs?
API Docs (Swagger) has a lot of overhead, and loads a lot of meta information in the background. It’s great for testing and exploring the available API calls, but should be used as indicator of performance.

I was told this was through their services to the database (we are using stored procedures.)

Thanks. But what tool/application/interface are they using to test those services?