Stiffer frames benefit the slow swinging player more than a fast swingspeed player.
When I hit with stiff big hoops, I get no more power than my Mfil200's. When I up the stiffness to 68, as in Aero500, I get no more power, usually less power.
When I used to borrow my buds Wilson2.6 grey stiffy, I hit softer shots constantly, with some going waaay long. He used my rackets, couldn't hit a penetrating ball if it landed beyond the baseline.
Slow swingspeeds get huge benefits from stiff rackets, and can't get softer flexing rackets to hit hard.
Fast swing speeds get little to no benefit from stiff rackets, but can make soft racket hit the ball with plenty of zip.