The option read size affects the overlap of disk reads/writes with network reads/ writes . If the amount of data being transferred in several of the SMB commands (currently SMBwrite, SMBwriteX and SMBreadbraw) is larger than this value, then the server begins writing the data before it has received the whole packet from the network, or in the case of SMBreadbraw, it begins writing to the network before all the data has been read from disk.
This overlapping works best when the speeds of disk and network access are similar, having little effect when the speed of one is much greater than the other.
The default value is 16384, but little experimentation has been done as yet to determine the optimal value, and it is likely that the best value will vary greatly between systems anyway. A value over 65536 is pointless and will cause you to allocate memory unnecessarily.