"It's not really that HTML5 requires extra work, it's that it lets you get away with not putting in extra work and then the lack of that work ends up biting you in the end."
I have never experienced such hard-torque spin in my life. It's not that you would need to put in overtime if you worked here. It's just that we really let you get away with never putting in overtime, if you don't want to. And then that lack of overtime ends up biting you in the end.
If you put in <x> effort into HTML5 you'll have a slow, shitty version. If you put in <x+y> you'll have a nice fast version.
If you put in <x> effort with a native app, you have nothing. If you put in <x+y> you have a working, fast app.
You get a working prototype faster with HTML5, but that's not a fair comparison with a native app which took more work to get working at all.
It's not just that JavaScript is slow, although that doesn't help - having to worry about the performance implications of using libraries rather than directly using browser APIs, or being forced to use native scrolling because JavaScript scrolling just can't avoid lag, is a waste of time. It's also that where native UI frameworks are designed for performance from the start - every single UITableView is drawn lazily (implementing "immediate-mode" infinite scrolling on top of the deferred mode DOM is awful), you can fluidly move between low-level and high-level drawing APIs (Canvas is a very sharp boundary), animation feels built in rather than a recent hack (and I hope you're not targeting anything other than WebKit) - in HTML you have to fight against an absurd layout system to do anything and, in some sense, work around it to do anything fast; there are usually many ways to do it but most of them are slow.
And it doesn't help that where native frameworks have built-in controls to do a lot of things - and those controls look and feel native simply because they have the home field advantage of being what everything else has to mimic - HTML5 is left with a bunch of shitty webapp frameworks and the option of reimplementing everything yourself. The creators of those frameworks seem to assume things like if the animation looks vaguely like the original, there's no need to actually get the details right to avoid uncanny valley, or make it performant at all, so you probably want to make it yourself. I hope you know what you're doing.
As much as I love HTML5, you have to fight every step of the way to make something that works right. Getting a prototype out is easier with HTML5 than native, but for a moderately complex app, making it actually work right is vastly harder.
(I do like the better layout systems and widget libraries of native frameworks - I did desktop app development with Swing and MFC before HTML/JS, and it really was significantly easier to put together a complex UI that matches the native look & feel with those. HTML5 gives you the opportunity to be creative that you don't have when you overrely on a framework, though.)