What is the point of attacking 20 times per second when mobs have half a second of damage immunity? I can see (as I often do) somebody hitting different mobs more than twice per second but I doubt anybody could actually deal much more than that while being accurate enough to hit separate enemies (indeed, this is the basis of my own "1.9 combat"-like system, where you can attack as fast as you want but you are penalized if you hit a mob that is damage-immune and the damage is less than the initial damage (vanilla itself allows the difference through if it is greater; e.g. a punch followed by a sword will together deal the damage the sword dealt by itself) or miss entirely), or that the game itself can even process that many (I'm not exactly sure but since the game runs at 20 ticks per second if it takes 1 tick each to recognize that you pressed or released a button the maximum rate it will recognize is 10 per second; going faster and/or not being in sync will actually make it worse as it misses activations. Of course, an autoclicker may not have this issue but then you are cheating, and this is likely why Mojang decided to make the cooldown work the way it does, instead of something like my system, since it can easily be bypassed by only attacking at the right times).
Fun fact: prior to 1.7 weapons lost durability every time they hit a mob, even while damage-immune or performing the death animation - so you'd lose durability at the rate of 20 per second, or 10 times faster than if you dealt as many attacks as necessary by e.g. a Sharpness V diamond sword, which kills most mobs in 2 hits (Mojang considered this to be a bug and fixed it in 1.7 but I see it as punishment for spam-clicking; I've watch people play on older versions and go through an entire wooden sword after killing a few animals, when it should be able to kill 30). This also costs hunger as well (not sure if it still does).
Also, this command given by the Wiki seems to indicate that there is no limit on attack speed, other than that imposed by the game itself for reasons mentioned above (for comparison, swords default to 1.6 for a 0.625 second cooldown; 1000 should thus give a cooldown of 0.001 seconds. You can also apply this command to a weapon instead of shield; I believe in this case any weapon you hold while also holding the shield has the attribute applied, but then you don't get the effect if you switch to a different item, like torches or food):
Also in case you want the 1.8 attack style try this.