Is it good to finish a PhD fast? I am not going to give details about the situation (for obvious reasons) but here it is: I got accepted for PhD to a very good American university, in experimental particle physics and I met a famous professor here who is willing to be my mentor. 

I talked to some of his old students and they all said good things about him (both personality related and research related). 

The thing is that his students usually finish the PhD in about 3 years (this includes taking classes, written and oral exams), while the average here is about 6. I am not sure if I should go for it or not. 

I really like the work that he is doing and a recommendation letter from him would definitely mean a lot and I am totally willing to put as much effort as necessary into the projects he gives me. I am just not sure if a 3 years PhD would be good for my future career. 

Will I get to learn enough? Will the post-doc or professor positions I will apply to might view such a short PhD as a bad thing (the students I talked to graduated quite recently so I am not sure about the long term impact of this)?
One of the questions is why his students finish so fast. That's likely at the core of whether or not things are "Good" or "Bad".

For example, some potential good reasons:


His lab has stable funding, which means his students need to take fewer TA positions to fund themselves, and can work on research instead. Note that this might also be a bad thing if you want to accumulate teaching experience.
His lab has a productive experiment and data generation pipeline, meaning his students might have clear experimental setups, there's a lot of institutional knowledge, etc. that prevents them from going down unproductive rabbit holes.


In both cases, a "Good" lab might have removed some of the waste time from doing a PhD that many people experience, which will cut down on the time it takes, potentially by quite a bit.

Some potentially bad reasons:


The course of study for his students is expressly just what's useful for their work in his lab, discouraging picking up depth or exploring interests.
The projects are "gift wrapped" and don't involve much thought on the part of the student as to how to conduct them - all they have to do is pull the trigger. This seems like a good thing, but can leave you with a lack of skills in articulating an independent research agenda later.


Just knowing the time it takes can't tell you which of those things (and others) is actually in play.