What does it mean? What do I do about it?
The -Mm parameter specifies the network message size. Larger messages can contain more data, resulting in less "chatty" communications and improved performance.
Increase -Mm to at least 8192. The maximum value is 32,600.
Before OE11.6, this must be changed simultaneously for both the client and the server. This is a hassle, but the payoff can be substantial.