Let's start with the uncomfortable truth: GWT compiler performance is really lousy. You can use some hacks here and there, but you're not going to get significantly better performance.
A nice performance hack you can do is to compile for only specific browsers, by inserting the following line in your gwt.xml:
<define-property name="user.agent" values="ie6,gecko,gecko1_8"></define-property>
or in gwt 2.x syntax, and for one browser only:
<set-property name="user.agent" value="gecko1_8"/>
This, for example, will compile your application for IE and FF only. If you know you are using only a specific browser for testing, you can use this little hack.
Another option: if you are using several locales, and again using only one for testing, you can comment them all out so that GWT will use the default locale, this shaves off some additional overhead from compile time.
Bottom line: you're not going to get order-of-magnitude increase in compiler performance, but taking several relaxations, you can shave off a few minutes here and there.
Unfortunately no, at least not out-of-the-box. Vaadin uses a server-side architecture, which means that all the widgets' states are stored on the server and not in the browser's memory, hence making the widgets dependent on the server-side. The communication between a widget's client-side and server-side is implemented deep in the core of Vaadin, so I think it would require quite a lot of work to get the widgets to work in a pure GWT environment.
Best Answer
In GWT application logic is normally run on client side. It only calls server when it needs to read/save some data.
In Vaadin application logic is on server side. Client side must normally call server after every user interaction.
GWT advantage:
App logic (replies to user interaction) is faster as it is run locally in the browser. It's also relatively insensitive to bad network conditions. Network is used only when needed (to read/save new data), which saves net traffic (important for high traffic sites).
In this regard Vaadin is slower and introduces a lag in UI interaction which is annoying to user. If network is bad this will show in UI responsiveness.
Vaadin advantage:
App logic is run on the server so it can not be inspected by the user. Arguably (Vaadin claims) that makes it more secure.