The challenge with performance testing is that the tests run in a simulated environment.
Simulated environments are fine for other kinds of tests, such as system tests, since certain aspects are abstracted. Mock servers, for example, can simulate behavior similarly to production.
However, unlike in functional tests, validating the system's responsiveness requires to take everything in the environment into account. At the end of the day, applications are running on actual hardware, thus the hardware, as well as the overall situation, impacts the application's performance. The system's performance in simulated environments will never behave equally in production. Therefore, performance tests are not a reliable way of finding performance bottlenecks.
There are many scenarios where an application can perform much better in production compared to performance tests, depending on all the immediate and imminent influences. The HotSpot JVM, for example, performs better under high...