There is a cap to IAS but im not 100% sure on what it actually is, but as for crushing blow:
Crushing Blow
This is a chance of reducing a monster's health by X% in a single blow.
-Default: 1/4th
-vs. Players: 1/10th
-vs. Hirelings: 1/10th
-vs. Champions, Uniques, Bosses: 1/8th
-with missile weapons, default: 1/8th
-with missile weapons, vs. Players: 1/20th
-with missile weapons, vs Hirelings: 1/20th
-with missile weapons, vs Champions, Uniques, Bosses: 1/16th
-The life removed is also scaled with number of players. So that if a monster has 450% more life due to 8 players (or whatever the value is) the life removed is further reduced by the same amount (450% would equal multiplying by 4.5 so the life removed by Crushing Blow is divided by 4.5).
* It is possible to get up to 100% chance for Crushing Blow. Anything above 100% is discarded.
* Physical resistance does apply to Crushing Blow damage, but only if the resistance is positive.
* Crushing Blow Items stack in most cases.
* Since 1.10 Crushing Blow is calculated before your normal damage. So, before doing the damage that you would normally do, there is a chance you will reduce the health of the monster by X% then normal damage apply to the resulting lower life. If you have more than one item with Crushing Blow, the probabilities will be added together. There will be one random check for a Crushing Blow. There is no check for each separate item, so one can not get multiple Crushing Blows in one attack.
In short it reduces a given monsters hp by X. This means if you were fighting a boss with 30,000 hp if you landed a crushing blow his hp would be reduced by 3750 then your normal damage applies, so it is very very effective at bringing a big monsters hp down very fast. The effectiveness of CB goes down as the monsters hp does so once they get very low on hp the effects of CB will become less noticable.