passive intake area should be 3-5X the active exhaust area
screens/filters generally restrict area by 50%
If too much humidity locks up in carbon, the water clogs it. You can drill the rivets out of the filter, put the active charcoal in an oven for a while, then put everything back together with rivets or screws. Typically active carbon when used properly lasts a while. High humidity can clog it which in turn, clogs the fan. Fucking with fan speeds can definately back up humidity in the grow chamber and then in turn clog a filter.. also it is good to use seperate filters for veg/bloom
To test if the filter is fucking up your fan speed.. simply seperate the filter from the fan... if the suction greatly increases (match experiment) you know the problem is @ the filter not the fan