I saw someone describe Jose Luis Lopez's activity level as working at "glacial speeds". And it's pretty accurate. Maybe the least active puncher I've ever seen.
Right now: Arthur Abraham, he only fights 1 minute per round and covers up the rest of the time, he also gives away the first 3-4 rounds figuring out his opponent but gets stronger as the fight goes on. But each punch is thrown with very bad intentions and caries his power into the 12th round.
Eddie Mustapha Muhammad comes to mind. Even in some of the fights he won it looked like he could've ended things a lot earlier if he'd just pull the trigger & let that right hand go.
Juan LaPorte was another guy who seemed genetically incapable of throwing more than 10 punches per round