I just posted something about pricing the gloves in Frogboy's thread but a cap on speed reduction is a good call. Or for sake of dev resources just say that slow effects cannot stack or even proc/replace when other slow effects are active.
So Wyrmskin Gloves will slow for full duration but if you keep hitting and gloves proc again the effect will not activate if the target still has previous slow. If you are slowing with a spell you will have to time the spell to activate after previous slow dissipates or waste it. You bring a little skill into the dynamic this way.
Actually I'd like all control spells to look like this - so a stun effect cannot replace another stun effect, must be timed to take effect after previous stun has dissipated. I'll make a post about this ^^