So I do understand why it got that reputation. But I'm using Javascript's raw performance in 2012 as an objective point of comparison. Consider a browser game. Given well-written JS coded with a competent understanding of the engine architecture versus well-written Flash, rendering the exact same animations, effects and interactions, you would find that the framerate in Flash could stay 25-30 fps on an iPhone 5, and be < 10 fps in Javascript. And that was with an alpha version of the Flash plugin. Similarly for JS bundled in Electron versus AS3 in the AIR engine as a "native" app. I know, because I wrote one of the first game engines for Javascript that attempted to do a subset of what could be done in Flash at the time.
What did make the dev process in Flash truly accessible was that it allowed the artists' pipeline to work in the same environment as the coders'. That did sometimes give rise to poorly written apps, but the fact is that the same thing in JS would have been even slower and more battery-draining, in an apples-to-apples comparison.