The last two patches buffed the heavy attack speed of the mk 4 cleaver and mk 3b bully club to usable levels but certain other variants of these weapons still have the slow versions of the attacks making them not worth using over those that got buffed.
The speed of the first heavy attack of the mk 2 club compared to the 3b is a good example of this since they both use the same damage profile but mk 2 is much slower, the mk 6 cleaver heavies are also painfully slow compared to the mk 4.