Flash
I think Flash is still your best choice, and probably will be for at least 5-10 years. Usually its not Flash itself that is "slow" or "stuttery", but it's the developers. Flash Player with AS3 is actually extremly fast.
Although, Flash definitly has its stregths and weaknesses, and I usually try to use it for what its best at. And that is throwing around vector/bitmap graphics in 2D.
Unity3D
If you want fast 3D in the browser I would strongly recomend Unity3D. There are a bunch of games and tests you can see live around the net. Sure Unity is a plugin that new users will need to download and install, but there are a lot of benifits.
One of the biggest benifits in Unity, is that you are using Mono Which is "An open source, cross-platform, implementation of C# and the CLR that is binary compatible with Microsoft.NET" that can compile to almost any platform (including Windows, Mac, and even iPhone). So you can basicly write once - deploy everywhere.