There are several things that affect the latency. You will get about 200us latency on program on a die, but that can be reduced by using some caching and acknowledging the writes before the program finishes, but that cache can be saturated by sustained writes. Especially with random writes that get high level of map updates. As that fills the write latency on sustained random writes will eventually climb to the 2 program instructions latency, which is 400us. With multiple controllers, only every third write is going to a specific controller. The latency will go down as you don't sustain writes to each controllers at the level to flood the caching.