But at the moment performance is poor in all browsers,
Any evidence of that? Poor compared to what?
and there's every reason to suspect the problems with ****** HTML5 *code* which will still be bad irrespective of browser will be just as bad as it was on Flash.
I haven't seen a single reason to suspect that.
And I can't switch to another browser in all practical terms on the iPhone or iPad.
True. But that doesn't change my point. Flash has no significant competitors for flash content on any major platform.