Malphite + Frozen Heart + Randuin's is hilarious.

Whoops. Thinking of the absurd movespeed slow on Q with blue smite and Rylai's

Doesn't work like that actually, there is no cap on attack speed reduction except that it is multiplicative, which means the more you get of it, the less each instance actually does. If you start with 100% of your normal attack speed, and malphite uses E, you're down to 50%, so his E is completely effective. after that, the Aura slows that 50% of your normal attack speed by another 15%, you get 42.5%. Add on the next 15% from Randuin's and you get down to only 36.125% left. If it worked additively, then he would actually reduce attack speed down to 20% of normal with that setup. Basically, you can never reduce someone's AS to 0 unless you have a single 100% attack speed slow (also known as an entangle, like Amumu's ult), but hypothetically you could reduce it to an infinitely small amount with enough stacked debuffs.

Attack speed reduction is specifically not counted as a standard slow, and is not effected by the same soft cap, or even tenacity unless the ability with the AS slow has another CC effect that is effected like tenacity, such as Nunu's snowball or Nasus' Wither.