Not having much experience with developing native mobile apps, I can only say what a pain it is to develop non trivial HTML5 apps that need to support a wide array of browsers. Your initial euphoria of 'oh wow I could make this thing for [insert browser of you choice] in less than [x amount of time]' is quickly mitigated when you try it on other browsers. Different browsers _will_ vary greatly in performance for different approaches and at least one browser _will_ have a bug. Fixing these performance issues (and not killing performance on other browsers again) and finding workarounds for bugs will then take 80% of the entire development time.

Having said that, things have improved the last years, especially when you don't need to support older android versions (> 4). However, performance will always be suboptimal and I found that people who claim the opposite simply have a lower bar for what good UI performance means - as harsh as it sounds. There is also a great number of tips and knowhow on how to make 'blazing fast' HTML5 apps, which people are quick to point out. Many of these help, but they don't always work for all browsers and they potentially limit you in what you can achieve (my favourite: don't use images. But what if the whole point of my app is to work with large images!?)