Another thing to remember, is that "large" and "jumbo" shrimp are different sizes between different supermarkets. I find it more accurate to describe shrimp size by their count-per-pound, which is usually expressed as a fraction, like 41/50, 21/25, etc. This is a range, and you can expect to have anywhere between the two numbers of that size shrimp in a pound, so 41/50's would have somewhere between 41 and 50 shrimp in one pound.
Also, there is the issue of how big of a volume of cooking liquid you have. If you have a couple gallons of water at a simmer, when you toss in say a pound of shrimp, it will cook faster, due to the high "stored heat" volume in the water, than tossing the same pound of shrimp into one gallon of water at a simmer.
At work, we often cook two pounds of 8/12 count shrimp (that's shrimp that's about 2 oz EACH, their huge, about as big as you can get), from frozen straight into the poaching liquid. It usually takes about 5 minutes, sometimes longer. I usually have about a gallon of water in the boil, and leave it on a low heat when poaching. I also stand over it like a hawk, watching. If it starts to bubble, off the heat immediately, drain it, and ice-bath it to stop the cooking.