The lines on the right are for gringos only. Non-US citizens have to use the other lines, which is why they are so slow. Plus since you enter the area from the left side, many gringos don't realize there are those faster lines past all the people in the other lines and they just get into the first line they see. As others have pointed out going through immigration and customs (if you're a US citizen and/or not on some list

) is usually very quick and efficient Customs they usually just wave me through or just scribble a check mark on my declaration.
The big questions have nothing to do with the US Govt. Will you plane leave on time? Will your airline process any checked luggage quickly enough? Will you go the wrong way to your connecting gate once you exit customs and hit the construction, which seems to ALWAYS be going on at that airport (and at most airports for that matter)?
More importantly, did you book the flight through an agency or directly with the airline and are all your flights on one set of coupons? If so they would not have scheduled it that way if it were not sufficient time. And, just as significantly, if for some reason, you are not able to make the connecting flight due to a late arrival in MIA or an unusual backlog in I&C, you will have much more recourse with the airline in terms of getting you on a later flight (w/o any price penalty) and/or providing you with some compensation for your inconvenience (if they're on seperate tickets you're SOL).