Bicycle Tutor

I am not a bike mechanic by any means. I was wondering if a brake cable can be too short and if the lack of slack can actually affect the braking. A mechanic saw my braking cables and immediately told me that I don't have total braking power. If that is true, why?

this can happen if the loop from the levers to the first stop on the frame is too short, it can cause problems due to lack of feel and unintended application of the brakes when turning the bars.
It should be a nice radius, allowing the wire to move smoothly inside the outer cover, slack is not required as this can allow the cable to jump out of the stops and result in no brakes at all.