We certainly have plans for performance tuning. We have done a bit of it already, but we have postponed doing so until the functionality is mostly present and working properly.
Regarding JavaScript performance, I have had the opposite experience. I assumed that the performance would be abysmal, but have often been pleasantly surprised. I can see that there are limits to performance to which I expect all JavaScript and Canvas implementations will be constrained. But diagrams that people are expected to understand and possibly edit cannot be too large, so it often works out OK.
So I actually don’t think performance is the immediate limitation for developers. I think the “looseness” of JavaScript, and in particular the creation of properties just by setting them, limits the usefulness of compilers and other source-code analysis tools. This means that individuals can create reasonably small apps, but it is much more difficult to create bigger ones. Inasmuch as apps are just UI front-ends for server functionality, that might be OK for a lot of cases. Although initial productivity is probably higher than for traditional languages/environments, I think the long-term productivity has a lower ceiling.