APM is actions per minute. From what I've heard the've limited the number of actions that can be done per tick to 3, I'm not sure what it was before but it can be rather limiting at times.
The reason behind reducing the default limit was to hinder bots and to reduce server load (bots were running large scripts which, in return, caused lag).Three actions per tick was chosen as, from data
that was the maximum number of actions that the majority of the playerbase could feasibly manage without extra "help". Our engine team are currently considering whether to increase the number.