A memory controller includes a buffer memory and a microprocessor. The buffer memory includes at least a first cache memory and a second cache memory. The microprocessor is configured to control access of a flash memory device. The microprocessor is configured to obtain a number of spare blocks of the flash memory device corresponding to a first operation period, determine a write speed compensation value, determine a target write speed according to the write speed compensation value and a balance speed, and determine a target garbage collection speed according to the target write speed. The microprocessor is further configured to perform one or more write operations in response to one or more write commands received from a host device in the first operation period according to the target write speed and perform at least one garbage collection operation according to the target garbage collection speed.