It's not really that that useful to talk about "ARM devices" as a class as they vary wildly in capabilities and this will lead you to make very misinformed, or in this case, totally wrong statements.

The i.MX6 has a VPU that supports h.264 1080p60 decode and 1080p30 BP encode. For more information about the VPU you can mouse over the block diagram: http://www.freescale.com/webapp/sps/site/prod_summary.jsp?co... or you may want to review the i.MX6 GStreamer docs as well: https://community.freescale.com/docs/DOC-93387