I went and dug up[1] the flying dragon .blend file from the old 2014 OpenSubdiv demo[2] and it works perfectly in 2.79a giving 30+ fps at subdiv level 4 when OpenSubdiv is enabled in the modifier.
But in recent 2.8 builds I just can’t get it to go fast no matter what I try.
This is on a Windows 10 laptop with an Nvidia 1060, settings set correctly to the best of my knowledge.
Tomorrow I’ll try building a 2.8 version from the old .blend assets on the grounds that maybe something in the old file is preventing it from working in 2.8 (I tried recreating the subdiv modifier with no luck).
[1] https://download.blender.org/ftp/sergey/dragon_adult_flycycle.blend (this link is long dead but archive.org can pull it up for you)
[2] https://www.youtube.com/watch?v=dzIl_S-qHIQ