From the wiki page:
"Attack speed reduction duration is not affected by tenacity. However if attack speed reduction is in one ability with crowd control like Nunu's Ice Blast, then tenacity reduces its duration. So attack speed reduction is reduced by tenacity only along with other crowd control. "
That's really interesting, I've never really thought about how that worked before. Another reason why Nunu rocks as support!
--
~Peaf~
"Later that day, I f***ed a panda." - PSO