Golf ball speed and backspin are inversely related when it comes to cover thickness. Thick covers produce less ball speed but more backspin and thin covers have more ball speed but less spin.
Since the cover is a lower “C.O.R.” material than the core, golf balls with thick covers also typically have lower ball speed.
Greenside spin is generated by maximising the contact surface area between the golf ball and the clubface.
Thick covers can deform more at impact and therefore contact more of the clubface so more spin is generated with golf balls that have thick covers.