Here is a tip that I found on other blogs and I think is worth mentioning again. I tried it, it reduced the number of permutations from 15 to 3 only (1 permutation for IE, 1 permutation for the FR locale, 1 permutation for the EN locale). That means a compilation time of 1:05.750s instead of 2:02.828s.
In the module file (blabla.gwt.xml), you need to add this line :
<set-property name="user.agent" value="ie6" />
This of course will produce permutations for Internet Explorer 6 only.
There is also another property to define the locales :
<extend-property name="locale" values="fr" />
So this will generate 2 permutations only : 1 for IE and 1 for the FR locale.
Here is a list of user agents :