Primary reasons for poor performance (in approximate order of importance) are:

1. Old, hardened brake pads. Replace with Kool Stop salmon pads.
2. Too much friction in cable housing. Replace cables and housing with Jagwire or Shimano lined housing.
3. Brake pivots corroded or dirty, resulting in high friction. Take apart, clean, grease if appropriate and reassemble.
4. Dirty rims. I use one of these to clean all the rubber off the rim braking surfaces: https://www.amazon.com/Swiss-Stop-Al.../dp/B00EN1UOU8
5. Straddle cables incorrectly adjusted, resulting in poor leverage. Several youtube videos can be found regarding how to adjust this.

There are other possibilities but I think these are the main ones.