Performance testing examines how the software performs (normally "how fast") in various situations.
Performance testing does not just result in one value. You normally performance test various aspects of the software in differing conditions to learn about the overall performance characteristics. It can well be that certain changes will improve the performance results for some conditions (say a powerful laptop with a fiber connection) and greatly degrade the performance for other use cases. And often the software can be coded to attempt to provide different solutions under different conditions.
All this makes performance testing complex. But trying to over-simplify performance testing removes much of its value.
Another form of performance testing is done on sub components of a system to determine what solutions may be best. These often are server based issues. These likely don't depend on individual user conditions but can be impacted by other things. Such as under normal usage option 1 provides great performance but under larger load option 1 slows down a great deal and option 2 is better.
Focusing on these tests of sub components run the risk of sub-optimization, where optimizing individual sub-components result in a less than optimal overall performance. Performance testing sub-components is important but it is most important is testing the performance of the overall system. Performance testing should always place a priority on overall system performance and not fall into the trap of creating a system with components that perform well individually but when combined do not work well together.
Load testing, stress testing and configuration testing are all part of performance testing.
Continue reading about performance testing in the Hexawise software testing glossary.