I tested a 64-bit build of LibreELEC and it does improve VC-1 playback.
Hopefully LibreELEC (Orion) 12 will be 64-bit by default.
LibreELEC 64-bit sounds pretty cool indeed, but in the meantime VC-1 playback has improved in my LibreELEC 11.0.1 fresh installation. It's not perfect yet, not smooth enough. But thank you developers for that improvement.
Previously I tried the overclock option with LE 11.0.0 installed over LE 10.0.4, but no big deal. I still got dropped frames.