I think JSPM should stress the part about bundling. It's the second person that I see who says "JSPM is slow" and the reason behind this slowness is that they didn't bundle the files or bundled them incorrectly.

We do the same in our project: we bundle all the 3rd party libs when they change, so only our own code gets retranspiled on each page load. During deployment, we bundle all the JS in a single bundle (which is good for us, as we don't have too much JS in that project).