My best guess from the (not much) information available here is the device has a buggy GPU driver that kills performance. It's pretty rare these days but is usually the main reason you end up with poor performance in strange circumstances. If you test a variety of other devices, I would guess the other devices would all be just fine, and only your specific device has an issue.
If you only have one device to test on, and it's got some kind of problem, it's easy to imagine that all devices have the problem, or that it's Construct's fault, or HTML5 sucks, etc. This is why it's important to test on a few devices, to make sure you're not misled by specific-device issues. Android in particular is a bit of a minefield with lots of quirky different devices around. And if I'm right about the GPU driver issue, ironically that is to do with the native technology on the device provided by one of the manufacturers - and nothing to do with your game, Construct, or HTML5.