In my experience street-wise spotters straight from school with a bit of maths and physics were the easiest. Those with an 'A'-level scrape or two in a maths and/or a science usually meant that they were self distracting seventy percenters which often meant they had a great spare capacity and greater ability to scan and rapidly resolve. Plus they just got stuck in and seemed to enjoy the learning.
I found u/ts (and I've had a few....) with any sort of further ed very hard work. One doesn't need to be very clever to do air traffic. The decision-making is very binary!