There is no hard cap, but obviously a practical cap based on the base speed of weapons and max values of available IAS bonus.
Fastest base weapon speed: 1.50 (daggers) or 1.60 (1H crossbow)
IAS from 2 rings + amulet: 9 x 3 = 27%
IAS from the weapon: 11% (rare/magic), or 20% (superior quality white items)
6% from legendary items in various slots:
Beckon Sail or
Tal Rasha's Guardianship
Andariel's Visage or
Mempo of Twilight
Off-hand: 20% from
Dead Man's Legacy for Demon Hunter off-hand
There is also the
Shrine of Frenzy which adds a 25% IAS for 2 minutes.
So if you just want the theoretical max weapon speed:
- Base weapon aps of 1H crossbow: 1.60
- Bonus for superior item quality: 20%
- Total bonus from equipment in general: (9 * 3)% + (6 * 5)% = 57%
- Additional from Dead Man's Legacy for DH: 20%
- Shrine of Frenzy: 25% (not sure if this is additive with items or multiplicative factor)
- Focus Mind passive from Enchantress: 3%
Max bonus from items for DH: 57% + 20% = 77%
Max IAS incl shrine = 102%
Final weapon speed: (1.60 * 1.20) * 2.02 = 3.8784 aps
I did not include Focus Mind as I don't know how it will apply. This also excludes the effect of skills that add IAS like Barb's Frenzy. (A Barbarian would have to use daggers instead of 1H xbow).