> I also get the feeling that desktop browsers (chromium, firefox) aren't very well optimised for ARM.

Freescale provides hardware video decoding support for Chromium's GPU media stack[1].

chrome://gpu snippet on my Sabre Lite running Chromium 37.0.2062.120:

Graphics Feature Status

* Canvas: Software only. Hardware acceleration disabled

* Flash: Hardware accelerated

* Flash Stage3D: Hardware accelerated

* Flash Stage3D Baseline profile: Hardware accelerated

* Compositing: Hardware accelerated and threaded

* Rasterization: Hardware accelerated

* Threaded Rasterization: Enabled

* Video Decode: Hardware accelerated

* Video Encode: Hardware accelerated

* WebGL: Hardware accelerated

[1]https://github.com/Freescale/chromium-imx