In our experience html5 canvas (in general, not only in OpenFL) has a problem with vectors or mainly with texts.
Testing in several frameworks the result is similar: complicated to get quality text on any computer or device (although you can) and complicated to get the application to maintain a good framerate when any type of text is included.
In the end we end up converting texts to bitmaps (when possible) to get smoothness in the framerate ...
Now, when there are dynamic texts (as is the case of some components such as FeathersUI), I suppose we should follow another type of strategy.
I guess Josh, that your components are using pure text (no bitmap fonts) and as you say may not be fully optimized yet.
I've tried the FeathersUI demo on OpenFL, on Chrome on an Android (motog5) and the scroll list gives the impression of sluggish. What I could expect considering what we already know ...
But, what has surprised me is the pixijs framework. I understand that it is the most optimized one out there, but there has to be a more "simple" reason. Is it possible for pixijs to use bitmap fonts directly? Perhaps what it does is convert normal text to bitmap fonts at runtime ?.
What I mean is that by testing different UI components based on pixi, it is smooth on the same hardware (60fps). Example:
The texts on my computer appear pixelated, which makes me think about my hypothesis (that the texts are bitmap fonts internally).
I'm talking all the time about the html5 canvas target. In any other OpenFL target, in our experience, everything is much faster even when there is text.