A master device has a buffer for storing data transferred from, or to be transferred to, a memory system. Control circuitry issues from time to time a group of one or more transactions to request transfer of a block of data between the memory system and the buffer. Hardware or software mechanism can be provided to detect at least one memory load parameter indicating how heavily loaded the memory system is, and a group size of the block of data transferred per group can be varied based on the memory load parameter. By adapting the size of the block of data transferred per group based on memory system load, a better balance between energy efficiency and quality of service can be achieved.