Im curios if there is much of an aero penalty with small fork mounted cages on a bike thats chugging along at 25kph for most of the journey.
There is, for two reasons:
Firstly, bikes rarely chug along at a steady pace. Nornally you go a good bit slower up hills and faster down them - and then you have disproproportionately more drag.
Secondly, it's not your speed relative to the ground that dictates drag, but relative to the air. If you go at 5 mph into a 20mph wind, then aerodynamics will make a big difference. If you go at 20mph the other way, it won't matter at all!
The rule is that slower riders save more minutes from aerodynamic improvements over a given distance than faster riders - because they are riding against the wind for longer.
But: you have to put gear somewhere and anywhere you put it has some sort of aero penalty. Bags on the forks isn't great aeroz, but might just be the easiest and most handy place to put stuff.