I am quite confused about this. The fastest possible way to rank up is to make one post every 2 weeks? So if I log in every other monday and make a post it would be the same as me posting a few times every day?
No. The fastest possible way to rank up is to make one post per day. The 'activity' score is basically the lowest value of your postcount and the amount of days you've been registered.
The second component, amount of days registered is only updated in blocks of 14 every 2 weeks. Not sure why, but effectively it boils down to one activity point per day.