The bench example writes 100 bytes at a time. The bench example writes a five MB file so 50,000 write calls are executed.
The maximum latency is the maximum time to execute this statement:
Where buf is a 100 byte array.
A latency of over 300 ms is unusually long. 150-200 ms is more common.
The maximum latency generally occurs when a SD card erases a large number of blocks of flash memory. It can also be related to wear leveling or remapping a bad spot.