For me it just feels better to do a clean wipe and flash with a major update like this, to start fresh with a "new" device. But I don't know if it actually makes any difference for performance and such. Maybe if you had a ton of crap installed.
Yeah, it's normal that it takes quite some time on first boot. This is due to the new runtime, ART. Whereas the old one, Dalvik, did just-in-time compilation every time you started an app, ART compiles everything ahead of time just once. This means apps take longer to install and take up a bit more space, but in return they start and run faster. This is why the first boot takes so long.
You don't need to reflash just to wipe, a factory reset will accomplish the same thing. But yeah, if you've changed other stuff in the system (through rooting) you can just flash the factory image to make it 100% stock again.
I thought the last preview was really smooth as well, but sure, it wasn't always absolutely perfect. Maybe it is even better now.