What I expected to happen was...:
Command Block would buffer, so commands could not be run too quickly. This would be similar to Dispensers and Pistons.

What actually happened was...:
Command Block can be activated as quickly as it can run commands. If setting a block adjacent to it as a block which produces redstone current, it will activate itself. If this is done by replacing the block currently in this location, it can be used to rapidly place Redstone Blocks, which can make commands run extremely fast.

Steps to Reproduce:
1: Obtain a Command Block. (/give @p 137 1)
2: Place it in an open space
3: Enter the command "setblock ~ ~1 ~ 152 0 destroy" into it
4: Power it from above
5: It will begin rapidly creating and destroying redstone blocks above it. When setting the redstone block, it becomes powered for a single instant, before it places another redstone block and reactivating itself. This cycles until the block receives a sustained signal to prevent it from detecting others. While allowing commands quickly should be kept, this quickly is definitely unintentional.

The above can be done without the block destroying particles and block items by changing "destroy" to "replace" and adding two command blocks adjacent to the redstone block which both set the redstone block space to air. It will still cycle as before.