In terms of development speed, which would be faster: webforms or mvc?
Some would argue Web Forms because of the "off the shelf" server controls that can do basically anything you want. Whilst this may have been true in the intial version of MVC, now that we are at v3 and the community is getting heavily involved - you'll be able to find a MVC-port (e.g custom HTML helper) to do those things for you.
Not to mention the vast scaffolding improvements in the MVC tooling will make your "CRUD" life a hell of a lot faster.
So it's hard to categorize "development", but in terms of getting a full page running based on a backend database, i'd say MVC is faster.
How about out-of-box performance between the two?
That's tough to answer. Anything with performance is. I've read somewhere that the Razor view engine is marginally faster than the ASPX view engine, which is what Web Forms is based on. Add onto that things like ViewState, and i'd have to say (without any backing "facts") that MVC would be faster. But don't look to the technology to have well performing applications, look to yourself to write optimal code using the technologies at hand.
In a long term which would be easier to maintain?
Definetely MVC - no question on this one. SoC (seperation of concerns) is a huge pushing point in MVC, not only with the M-V-C segregration, but also the "convention over configuration" approach that is at the heart of MVC. EditorTemplates and DisplayTemplates are a perfect example of this - they work off convention (the model type being passed to it). So you can create multiple templates and flick over different model types without any backend code changes.
That being said - nothing stopping you from having a well seperated and layed out Web Forms application - but there's only so far you can go without hitting a roadblock in the technology itself - such as the "code behind model" which cannot be avoided.
Overall - MVC FTW.