The architecture of the system, such as how it writes data iniitially and how it handles things like "garbage collection," plays the biggest role.
A lot of new all-flash arrays on the market perform quite well when they are new ("fresh out of the box") but will see a significant performance hit when all the cells have been written to and they are having to perform maintenance tasks (such as garbage colletion) for new writes. This drop is often referred to as the write cliff and the "steady state" performance can be as much as 50% lower than the "fresh out of the box" performance. This impact could be less if the system is under a light workload, but can be more substantial if the array is experiencing heavy write activity.
But in general, once you've reached your stead state performance, the system should be able to maintain that performance level over time.
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