This is related to the Undertow system and the way it relates to the XNIO worker. In addition to the below please see
IO Worker configuration for Undertow which explains in more details the settings within the IO subsystem.
If you look at the configuration for you ajp-listener you see a property called worker (see Wildfly Model Reference). By default the worker used is the default one found in your domain:io subsystem in the configuration file. You can easily add a new worker thread via the cli or the web console, and tweak the settings as needed.