I'm not sure which CPU you have specifically but the lowest-end model of the X1 Carbon Gen3 has an i5-5200U [1] that lists Intel Quick Sync Video support.
From the wiki page for Quick Sync [2]:
> Intel Quick Sync Video is Intel's brand for its dedicated video encoding and decoding hardware core. Quick Sync was introduced with the Sandy Bridge CPU microarchitecture on 9 January 2011 and has been found on the die of Intel CPUs ever since.
I can't confirm but I'd guess your performance issues lie elsewhere than in the h264 decoding specifically.
If you check out the generation-codec table in that wikipedia article [1], under Broadwell (I believe that's the 5200U's generation name), it says there is support for AVC (which I believe is H264, I'm not a codec wiz), so that's a really good point. I'm not sure why I've consistently had issues with this on my machine then. I wonder if this is something with a configuration on Linux then?
Thanks for pointing that out. I've looked at this table before and payed attention to HEVC, not AVC, so I believe that's where my mistake came from.
Accelerated video decode is often disabled by default on Linux versions of browsers and can be quite dependent on versions of drivers/mesa/X-vs-Wayland/etc.
From the wiki page for Quick Sync [2]:
> Intel Quick Sync Video is Intel's brand for its dedicated video encoding and decoding hardware core. Quick Sync was introduced with the Sandy Bridge CPU microarchitecture on 9 January 2011 and has been found on the die of Intel CPUs ever since.
I can't confirm but I'd guess your performance issues lie elsewhere than in the h264 decoding specifically.
[1] - https://ark.intel.com/content/www/us/en/ark/products/85212/i...
[2] - https://en.wikipedia.org/wiki/Intel_Quick_Sync_Video