frozen mallet on lee sin ?

Too expensive and you don't need the sticking power on Lee. If you feel you do, you need more practice on Lee. Not to sound overly harsh.

frozen malet is a 1.5 sec 40% slow on auto attacks ..

lees e is a scaling abilits 20/30/40/50/60% slow depending on level .. duration always 4 sec but you must hit both instances of your e in order to slow them ...

if you buy malet for slow, it is not a good option... if you buy it for 600 hp and 40 damage (tanky dd) it is a nice item

his E doesn't feels like its slow at all

you noticed that too? it's more like a brief root or so