Flex compiler does use lot's of memory and CPU resources, so does Flexmojos. There's not much new in here.
Some time ago I was reading this blog post with some memory parameter suggestion. And guess what, compilation becomes faster using it.
There are 2 ways to change maven JVM settings:
1 - Editing launch files
Edit mvn (non-Windows) or mvn.bat (windows) and add the following line:
$ set MAVEN_OPTS=-Xms512m -Xmx1024m -XX:MaxPermSize=256m -XX:PermSize=64m
On Windows you can add that on the first line and on non-Windows systems you must add that after "#!/bin/sh"
2 - Setting an environment variable
This process will vary from OS to OS.
Just add an environment variable with name equal to "MAVEN_OPTS" and the value equal to "-Xms512m -Xmx1024m -XX:MaxPermSize=256m -XX:PermSize=64m" both w/o quotes.
The main advantage for the second method is that it will still work even after Maven is updated.