What is RDBMS optimal performance? How do you define optimal performance? The people in the organization responsible for answering these questions are usually database administrators (DBAs), network administrators, or server administrators. At some level, this team might include software architects and software developers. It is not uncommon to see one person wearing some (or all) these hats, and at some advanced level it is as hard as it sounds (there is a reason for the big bucks paid to DBAs!).
Understanding what is involved in optimizing RDBMS performance would allow you to see possibilities for improvement and the limitations thereof. The ability to know one from the other comes with experience.
There are a few benchmarks that allow one to make an educated guess about whether your database server performs at top speed. Some of them are published by the vendors (and understandably should be taken with a grain of salt); some of them can be found on the Internet, published by aficionados of a particular RDBMS under unrepeatable conditions (and also should be taken with a pinch of salt); and your DBA can come up with some custom benchmarks, specific for your database.
Fortunately, there is an independent organization dedicated to benchmarking RDBMS performance: the Transaction Processing Performance Council or TPC for short.