This forum thread speaks a bit about the upcoming new "super dev" mode, with better tools for profiling.
Just a bit from it:
We're actively working on it. Note that 'super dev mode' isn't quite in
existence yet, it's more of a goal than a concrete thing at the moment.
Though there was a short lived branch with that name--most of those
particular changes have already gone in.
If you like, you can profile DevMode by running GWT from trunk and passing
-Dgwt.speedtracerlog="/path/to/myproject-st.html" which will cause DevMode
to output a SpeedTracer log to that file. You can then use the Chrome
Extension SpeedTracer to inspect the generated log to see where DevMode is
spending time for your app. If you'd like to share, you can feel free to
post screenshots of the SpeedTracer detail view for your Module Init.
Great fan of Open Source, Linux, and web development with GWT; all of these come together for my ESSENTIAL GWT book!