I've had Flash for like 5 years and this current project I'm working on is the first I've ever tried to stay in a mainly-FBF style, and I have to say, it's pretty damn hard if you're not used to it. The onion skin is a godsend like Sup said above, otherwise, Flash would be a nightmare. Just practice A LOT, walk cycles are a little tricky I've come to realize. As for lagginess, try drawing 12 or so frames a second (24 is the standard, not 25) or hell, even 30. I just stepped up to ensure fluidity, otherwise, with my previous tweenfest movies, 24 frames per second worked just fine. I guess try using even numbers for easy math i guess. Oh, also, do the FBF in a symbol, then use it tweened, like walking so you don't have to position a bazillion frames.