Her slow is ridiculous
If maxed second, which it is in almost all cases. And it costs a shit ton of mana, which given her small pool even at level 18 and the fact she is essentially forced into building CDR to keep a relevant E uptime causes her to spend mana a lot.
has an attack boost that's not only extremely effective and powerful
She builds Nashor's Tooth for a first item. Of course it´s gonna make it powerful and effective.
but also lasts 10 whole seconds.
Also one of the reasons why she has to build CDR. The downtime period of her E for those who don´t pack flat CDR(10% total) or CDR/level(20% at level 18) runes is large enough to allow punishment, the forcing of trades, etc.
Heck, even when packing those runes, the downtime is large enough. If you can´t do stuff to a Kayle in 6 seconds, you´re asking to get demolished.
But seriously, why is her boost 10 seconds long if it's gonna be so effective?
Because if it didn´t exist, she wouldn´t be viable anywhere, and if it were shorter, the mana cost would have to be reduced or her mana pool increased to compensate.