Is it good to finish a PhD fast?
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.