Without knowing the conditions of retrieval or the complexity of the data, it is not possible to say if the database performance will be sufficient. However, we can say in general using the Mendix DB will be faster than data communicated over APIs. There are fewer layers of delay between the Mendix DB and the Mendix App than with any other external DB. Instead, you should work on tuning your database and application to ensure acceptable levels of performance. This may include adding indexes to your database, archiving your data, ensuring your database is sufficiently sized, and optimizing the construction of your Mendix page.
For more information on how to make the right decisions for your situation, check out the following Mendix Learning Paths
Hope this helps!