i have a GXT 4.0 app on top of GXT desktop (i made it work in 4.0 by converting css to gss)
it takes to compile it ~1 minute with <collapse-all-properties />

if i add in gwt.xml
<set-property name="gxt.theme" value="blue,gray,neptune,triton" />

compile time jumps to 10-11 minutes