Having recently caught the Windows bug (I'm actually a Mac OS X and GNU/Linux user) again,
I decided to try and run a Pico-8 demo inside an XAML Webview container this morning.

While the setup itself was straight forward enough (I simply exported cast.p8 to html5 and made the generated cast.html the source for the WebView of an empty Windows 8.1 C# app template), the resulting performance turned out to be abysmal.

I get approximately 1/6 the framerate of the same file running inside Internet Explorer 11 (which rather surprised me, as I previously thought the WebView inside WinRT was merely a wrapper on top of the sandboxed Internet Explorer that is part of Windows Modern -- AKA Metro).
The same is also true for any carts I tested while surfing the Pico-8 section of this website, inside the WebView.

Strangely, most other HTML5 content works fairly well. Including WebGL content and, strangely (as I assumed this might've been the problem), other ASM.js/Emscripten applications.

Is there anything that can be done about this? Has anyone here tried porting their game to Windows 8.1/Windows 8.1 Phone before? The only "solution" I can come up with myself would be targeting Windows 10. But that would severely limit the potential player base.

PS: I've also tried running the demo inside a WinJS app. Same result and the debugger didn't reveal any errors short of WebAudio API related ones (WinRT prior to Windows 10 apparently doesn't support WebAudio).