Actually more accurately, as wind builds, on almost all boats there is a point where the boat becomes over powered. At that point, the drag of overcoming heel induced weather helm and the reduced efficiency of a heeled sail, results in a drop in speed. Initially, reducing the power of the sails will allow a greater speed. This does not mean shortening sail. It does mean flattening the sails and reducing the angle of attack.
Of course at some point in the wind range, on most boats it is necessary to reduce sail and depending on the boat this can result in an increase in speed.
More modern designs prefer to be sailed with smaller angles of heel, and especially reaching and beating can have really significant gains in speed when properly powered up or down. More traditional designs tend to build gobs of weather helm when overpowered but are less effected speed wise.