If <oXygen/> is used on large documents (more than 10 MB) and you see that performance slows down considerably after some time then a possible cause is that it needs more memory in order to run properly. You can increase the maximum amount of memory available to <oXygen/> plugin by specifying the parameters -vmargs -Xmx in the command used to launch the Eclipse platform.
The maximum amount of memory should not be equal to the physical amount of memory available on the machine because in that case the operating system and other applications will have no memory available.
The amount of memory allocated for the FOP operations is controlled by a different setting available in <oXygen/> Preferences: Memory available to the built-in FOP.
Modifying the value from 256 to 512 changes the memory available from 256 to 512.