a quick search comes up with the canal authority saying 16KPH, and also that the Ever Given has 2 bow thrusters of 3,400 HP each.
even a total main engine failure seems unlikely to me to have been the main cause, but im not a super container skipper. it clearly had some momentum when it plowed 40+ feet deep into the sand, and given the low speed, the ability to respond to rudder input would have been limited, I'd think. even 6800 HP of electrically driven bow thruster powered by 8 aux. generators might not have been enough to counter the drift from the crosswind by the time they noticed the deviation from center channel in a sandstorm, and were locked into a groove in the sloping bank.