Viewing modern large fan engines as having a core ‘motive power unit’ driving a large ducted fan; then when replacing the core with a large electric ‘motor’ and driving a conceptually similar ducted fan, then why expect a reduction in speed.
My assumption is that a ducted fan is more efficient than an open rotor / propellor.
The characteristics of an electric power unit might be easier to match to a fan - still requires gearing, thus aircraft speed / altitude could be better matched without the limitations of requiring ‘stable’ core airflow which has to pass through a fan.
The primary driver for cruise altitude is more likely to be the balance of engine thrust / weight, battery weight, solar panels, additional fuel, and design range / payload of the aircraft.
A larger choice of altitudes for best wind speed - reduced block time, higer = faster ?