Wookieepedia just spat at me:
Lucrehulks are almost twice the length of an ISD1 (3000m vs 1600m)
But half half the engine power (Atmosphereic Speeds of 500kph vs 975kph)
Of course, that doesn't say much overall.
But check yourself
http://starwars.wikia.com/wiki/Lucrehulk-class_battleship
http://starwars.wikia.com/wiki/Imperial_I-class_Star_Destroyer
Makes sense though. The CIS Lucrehulk's were just cargo ships they stuck guns on. The size and slowness makes sense for a massive cargo ship. And for the speed, anytime we see Lucerhulk's in the movies they seem pretty stationary
Their size and carrier capacity is probably what made them a viable battleship.