Why are external PhDs considered to take "decades"? I have just finished my master degree (TU Vienna) and I am working 20 hours per week. I have talked to several professors in my field of interest and they said that the earliest position for a PhD would be in December, because of funding.

However, I think I could take the route as external researcher and do my PhD by working 20 hours per week and devoting the other 20 hours to my research.

By browsing some posts here I have read that a lot of you consider an external PhD as inefficient (it takes a long time, etc.). Why? What are the reasons for that? Can't such a PhD be done within 3 years?


  UPDATE


PS.: What are mostly the reason that it takes so long?
Being a PhD student is a full-time job. 

True, "officially" you may be assigned to 20 hours of "study" and 20 hours of "work" by the university, but this is just a way to determine your salary, and does NOT reflect the amount of time you actually spend on research studies.

In the U.S., the nominal time for PhD is about five to six years average. This assumes the student is a full time student. While some people finish their PhD in three years or less, this is truly exceptional. The average is six. (This was the case at least in the school I attended, in the Computer Science department. Length may vary in other departments.) It also depends on what you aim for in your PhD: is graduating after publishing just one paper good enough for you? 

Now let's do the math. If it takes about six years to finish a degree, assuming full time, then how much time would it take for a part-time student? Take into consideration also 20%-30% extension of time due to context-switching, but also reduce 15%-25% of the time due to being more efficient  (it's easier to waste time when working full-time, in my opinion).

You arrive at 10+ years. 

Then again, I know people that worked part-time and studied part-time, and were able to complete their PhD studies in four to six years. I also know people that took five to six years to finish their MSc degree (which nominally takes two years), due to having another full-time job.. It depends mostly on your abilities and aims from PHD (but also quite a bit on your luck).