Okay, I understand that part. But say you were to do either of these...increase the pressure or increase the cfm.....which one would draw a vacuum faster? That's what I'm really trying to understand, what is the end result of the drawn vacuum in relation to the pressure and volume of air moving through the venturi?
What I'm trying to say is that, in order to change the CFM, you need to modify the venturi or pressure - you can't just say "change the CFM" and expect it to work. It's all immutably related, and you can't change one thing without changing another.
If you change the pressure, the CFM will change - that's the physics of the situation. If you want to change the CFM, you *must* change the pressure - unless you're going to modify the orifice on the vacuum pump. CFM is the flow rate directly and inexcricably dependent upon pressure and orifice size. If you want to change CFM, you must change one of those other items.
So, want to draw a vacumm very, very quickly? Increase pressure. HOWEVER, the venturi will probably need to be modified to accomodate the increased pressure, since you can easily overpower the valve because most are designed to operate within a set of specific pressures up to a maximum. Lower pressure will usually work without problem, to slow down evacuation, but trying to speed up evacuation by increasing pressure may simply overwhelm the venturi, and end up blowing motive gas (from your air compressor) into the chamber you're attempting to evacuate.
The actual application varies by what you're trying to evacuate - liquid, gas, or particulate matter. The document that Jeff linked to will contain a lot of information about all of that.
What's the ideal pressure? That depends on the specific venturi valve design. My suggestion would be to stick with the pressure requirements of whatever venturi pump you've bought. Increasing pressure slightly may show a faster evacuation of the chamber, but I wouldn't expect it to be able to cope with much beyond about 10% increase in motive pressure.