In fact you hit them easier as the leman russ will most probably sit in cover (just the tracks is enough to grant the save) while the raider has to jink.GW says speed is a weapon however you hit a raider that has moved 30 inches as easily as a lemun russ battle tank that has been in the same spot for 2 turns.
Speed was a defence in previous editions (mostly 2nd but at least fast antigrav also had defenses against melee in later editions) but today it is a bit of movement that gets outclassed by Drop Pods anyways.