The MB is an Auros B450, I get 28 MH/s from the card in the x16 slot but only 14 from the card in the x4 slot.
You don't have luck with the board.
the results now on my boards
Asus Z170 PRO GAMING (26,7+26,6+20,2)
Asus Z170 PRO GAMING (25,9+25,6+19,3)
MSI Z170A GAMING M7 (25,9+19,6+16,8)
MSI Z270-A PRO (27,6+20,6)
Asus P8Z68-V PRO (22+22+15,8)
AsRock A55 Pro3 (25,2+16,0)
AsRock 880G Extreme3 (19,5+19,5+15,2)
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Really good analysis ...