GWT on AppEngine (which it has great integration for) has a ten second startup time when not loaded, and on the free tier they unload it when not in use, so that's not the greatest.

I used GWT for a while and it would be great for enterprise and large teams. The static typing which I usually love/insist on though, does seem pretty cumbersome for something as simple as a web page. I've switched to scala play for the back end and lots of tiny angular 'apps' on the front, and it's so much faster to develop for.