Dual-wielding is potentially bugged

Earthshatter says 30% less attack speed while dual-wielding, but even the average damage per use goes down.
I think it takes the average of the weapons too, despite not explicitly stating that it does so?

Should be easy to test with a strong mainhand and a weak offhand. I get 43% less damage despite it saying 30% less attack speed only.

1.688.489.387 for reference bug, dunno if relevant.