The GWT compile process has been taking longer and longer, as I’ve moved from 1.0 to 2.0, because they keep adding optimizations and functionality. You could deal with this in a number of ways. You could write less GWT. You could buy a faster computer.
The way to target just one browser is to put this string in your module (.gwt.xml) file:
< set-property name="user.agent" value="safari,ie6,gecko1_8" / > (remove the spaces next to the angle brackets)
navigator.userAgent. It is not as fine grained as the ubiquitous browser detect script, but shows you what different browsers are known to GWT.
For other tips on speeding up the compile process, check out this series of posts.