Performance differences with ES3.1 and ES2

So now that ES3.1 can be used out of the box and my new PC can handle ASTC compression much better than my old rig, I gave ES3.1 + ASTC a spin, for Gear VR (using my S6).

Turned out ES3.1 + ASTC renders much slower than ES2 + ETC2. ~20 fps difference at times. Is it by design or is it something that is still being worked on as far as optimization go?