The reason it's "slow" is because you have the bow set-up for it's worst efficiency.

By turning down a 70 Lb peak bow to the low 60's you are putting the cam out of time drastically (And SL type cams are VERY sensitive to any timing changes), you are increasing the brace height, and not putting enough pre-load(or stress)in the limbs. All 3 factors combined add up to a poor performing bow.

By getting a pair of 60 Lb limbs (or "heavy" 60's, are available form Mathews if you want a few more pounds), you will get a quieter, faster bow that has a better feeling wall and nicer force draw curve.

My 28" cam Q2 at 76 Lbs ("heavy" 70 limbs) shoots a 435 grain Easton A/CC 3-60 arrow at 274 FPS w/ a fletcher tru-peep, Loop, and 2 Sims String Leeches. It will do 245 FPS w/ a 520 grain 2315.

JeffB