I didn’t say it wasn’t, I said not entirely.
As I wrote in my guide, the devil is always is the details but not everyone cares to look at them. Armor rigs tend to slow you down but not all of them. The active tank and the hull-buffer ones do not slow you down, they just increase the powergrid requirement of local armor reps.
If we are looking at the medium armor reps and for the sake of the example, we say a medium armor repairer would need 100 powergrid, a 5% increase of powergrid would mean a meager 5 more powergrid, resulting that fictional armor rep to require 105 powergrid instead of 100 - with the armor rigging skill at 5.
A Megathron with ion blasters and void L does around 1300 dps and still moves at 1250m/s.
Yes some (not all) armor ships are slow bricks but it is not the rule.