I don't find Flash slow or cumbersome; it's just buggy, inaccessible, rigid, and generally distasteful.
Can I set a custom font? Right click to save a linked image? Drag a block of selected text to save it, or from elsewhere into a text box? Nope! Those three points rule out decent AA (can't use Mona font, Flash doesn't recognize it for me), ease of access for an imageboard (can't save images quickly, have to go through some process to show the image in a separate window and then go to File->Save or similar), and kopipe for textboards (ok, maybe this one isn't quite as big a deal, if you're focusing on mostly "serious" boards, but it's still a usability issue and an area where Flash is seriously lacking).
Don't get me wrong; Flash does have its uses. Amazon's new player in the MP3 store is a hundred times simpler than downloading Real Player or Windows Media clips. YouTube et al. would certainly never have caught on without the Flash video streaming. And for animations and constant interactivity, Flash is much, much better than the alternative -- Java. Let's face it, Flash has pretty much entirely killed Java on the web.
I must point out, however, that most of these have one thing in common: they are all presenting multimedia. Flash video/audio streaming is clearly superior to Real Player, and Flash animations clearly beat embedded Java applets. On the other hand, using Flash for plain text and images doesn't have enough of a justifiable benefit.
I agree that it's a decent platform for prototyping, inasmuch as Visual Basic is an excellent environment to work out the surface details and basic feel of an application before actually writing it in another, more capable language. The latter part is the key; too many overgrown applications start in VB and outgrow their foundations, and what you're left with is a mess instead of a good design. The very same goes with prototyping stuff in Flash. It might be a quick way to get started, but after you've got a foothold on how it's supposed to work, perhaps Ajax would be more user-friendly. Maybe not as developer-friendly, but if you have a clear idea of how it's supposed to work, the design will be inherently cleaner (so it should be sufficiently easy to maintain).