The main problem with older Android devices is they have buggy graphics drivers that crash the system, so browsers have to turn off GPU accelerated rendering to avoid crashing. Then you end up with a slower software renderer. It's more of an Android problem than with C2 or Crosswalk, because the drivers need to be fixed... you can force it to use GPU rendering, but then you risk crashing the system (to the extent you have to pull the battery out and reinsert it for it to work - then think about people with phones with a sealed-in battery...!)
Newer devices should be fine.