Hmmm, hard to say without more information. Just after you get it started feel the battery terminals and both ends of the ground strap, if anythings warm that could be a poor connection.
Tried putting booster cables on it from another battery? if it still cranks slow it's likeley starter related, if it's magically cured it's battery issues.
Try connecting a jumper cable from the battery negative terminal directly to the case, if it cranks better it's the body to trans ground.
Turn on the headlights and try cranking it, if they go out and you know the battery is good check the terminals on the starter.
Good luck.