Gotta disagree with you there. Ruben would often (mainly, in fact) throw looping punches, but far from slow, in my opinion. Then again that may've had more to do with his timing and punch placement than speed. Still, I never noticed any speed deficiencies unless you're referring to him mainly in his latter days. I thought him quite quick in his prime, actually. Watch his rematch with Castillo. His lead left hook is a thing of beauty, and very quick off the trigger. A sneaky little punch that he was able to land frequently on an extremely smooth stylist. On the slower than average hands/quick feet front, I'd mention Luis Rodriguez. Another whose exceptional technique and awareness more than compensated for any short-comings that may've been caused by those long, ****** arms of his.
I'll throw a shout to Bazooka Limon as well. Terrible technique, punches telegraphed worse than a 1950's telephone, yet extremely effective all the same due to his clubbing power and indomitable will.
I dont see it, both arnt quick but they never striked me as being 'slow'. Jim Watt definitly needs a mention.