I've asked for clarification but it sounds like some device's GPU drivers are slow, and WebView uses a different rendering approach that runs in to the slowness where the browser doesn't. This isn't the same as saying the webview will always be slower.
AFAIK Chrome Custom Tabs show unavoidable browser UI, and you can already publish PWAs (web apps), but they don't appear in the Play Store.
If there were better workarounds we'd look in to them. As far as I can tell though there aren't any which don't have even worse tradeoffs.