Some questions about the time for a PhD program in the USA?
The requirements for a doctorate in the US vary both by state and by institution, but you seem a bit misinformed as to the general process. Some institutions have required courses and you can be asked to leave if you don't do very well. A C in a doctoral level grad course is like a D- or F in an undergrad course. It is not acceptable. One or two out of the set might be ok but more and you will not be invited to continue as you don't show sufficient seriousness. While the GPA itself may not be too important, you need to demonstrate that seriousness. 

Even if there aren't specific required courses (which are there to assure you have knowledge of the breadth of the field, primarily), you will certainly be required to pass Qualifying Examinations. There is usually a set of these (maybe three or four) and again they cover breadth, primarily, but at an advanced level. They are normally hours long. Fail any of these and you can (probably will), again, be asked to leave. The courses, required or not, prepare you for these exams. The exams are quite rigorous as are the prep courses. It is even possible to be asked, in an oral, a question on an unsolved problem, just to see how you attack it. 

The "sufficient amount of new research" is required, also, to be of a fairly high quality and you will need to show in this research that you have delved quite deeply into some, usually small, area of the field. This normally requires spending time in seminars with a professor (and group) who specialize in that area. All of this takes some time. It isn't just a paper count, but something that the faculty considers significant. 

It used to be regularly doable in about four years, but that has been extending for quite a while. Often this is partly due to the fact that students support themselves working in labs or teaching, which, again, takes time and adds to the total. If you are self funded and can devote yourself full time (60-80 hours per week) then you can shorten the time. 

My doctorate had two requirements. Pass the qualifiers (both written and oral) and write an acceptable thesis. I don't think I knew anyone who did it in four and this was many years ago. No professor will want anything to do with you unless you show seriousness of purpose. Poor grades and low expectations won't get you there. It is quite a bit more than putting in minimal effort and checking a few boxes. 

On the other hand, it can be exhilarating to be associated with a community of scholars, both helping and being helped by smart and dedicated people. 

Every once in a while an Einstein or Feynman comes along and if they are lucky and recognized early, may have a shorter path. But likely not. Seriousness of purpose.