The block size parameter determines the time resolution of the compressor’s source loudness measurements. Generally speaking, lower block sizes will give more accurate results. However, lower block sizes also require more CPU resources, so it is necessary to find a balance. You can squeeze improved quality and/or performance out of the compressor by tuning this setting based on your available CPU resources.
Fast attack and/or release times may benefit from similarly low block size.
BPM units for block size are evaluated once at the time you press Apply.