I work in an outpatient specialty clinic where appointments are scheduled every 20 minutes, 3 per hour. I'm a nurse's assistant and I've worked there for over three years. The doctor who runs the practice is an excellent doctor and his patients love him. However, it is common that we are running late (usually about 20 minutes but up to 45-60 mins on the worst days), especially later on in the morning or afternoon session. I personally always hated having to wait at a medical clinic and often bemoaned the apparent unfairness. Having worked behind the scenes for a while now I will say this:

The vast, vast majority of times (like 95%) that we get way behind schedule are due to two things:

1) Patients showing up late. If you are a new patient at a clinic you should expect 10 minutes of paperwork to complete prior to being ready to see the staff. Even though we tell patients this when they schedule the appointments, most show up right on time and then immediately put us back those 10 minutes spent on paperwork. Others will simply show up 15-30 minutes late for a 20 minute appointment, the math just doesn't add up - we're going to be behind schedule. The absolute worst is when the 10:00, 10:20 and 10:40 patients all show up at 10:20, screwed. This happens often.

2) The doctor I work for has a policy that he will address all the questions his patient has. Some people just don't know when to stop and will ask question after question after question leaving the appointment to run upwards of 40-45 minutes. Everyone after this person again, is screwed. You could argue that the doctor made his own bed with this one and that's fair but he is trying to practice medicine the way that he thinks is best for his patients and he ultimately has decided that answering all questions is more important to him than maintaining a tight schedule. BUT - it is never the truly sick patients that take advantage of his time like this. This is one thing that I think it's difficult to appreciate as a non-medical person. When a patient is legitimately ill (either acutely or chronically) with true pathology, the appointment is incredibly efficient. The doctor will notice it right away, zero in on it and act accordingly. It's very difficult for a lay person to understand how easy it is to determine whether a patient is truly ill or just experiencing normal, transient symptoms. I have very minimal medical training, enough to satisfy my job requirements but nothing like what a doctor or nurse has, and I can tell you when something is wrong immediately upon taking a patient back to a room. I don't mean to at all downplay any person's symptoms, I fully believe how frustrating they can be but there is a very real difference between experiencing symptoms and having a definitive disease and most importantly there is a major difference in what medical professionals can do to help people in this scenario (see below). I mention this caveat because the patients that take forever with a million questions are always the ones that are there because they are experiencing symptoms but don't have any definable disease, like: they have a belly ache for 5-10 minutes once every two weeks; or they have had diarrhea once per week for the past three weeks. (I work in a GI clinic if you haven't already guessed) But these people worry, Google their symptoms and come in with a hundred obscure, impossible diseases that they think they have. And they want the doctor to explain why they don't have each one. Or they want answers to questions that medicine just doesn't have. There are a million theories out there (especially about diet, vitamin intake, etc) but not a lot of definitive, time-tested data. There's a joke in medicine that given enough time, every theory will eventually be replaced by a new one. Unfortunately, there is a limit to what the medical field knows and what it can help you with but most people don't realize this. We only have so many tests and so many ways to investigate your symptoms, but these tests are all the ones that WILL find serious illness if it is present. People don't want to hear that "this is just the way your body is functioning right now - it likely won't be like this forever - but dealing with occasional diarrhea or bouts of constipation after 3 days on planes or a baby that spits up after occasional feedings is completely normal". Rather they come in seemingly expecting to walk out of the clinic with an answer like: "the reason you have diarrhea twice per month is because of a rare macronutrient absorption issue". No one knows the definitive answers to these things and if your doctor is telling you otherwise he/she is guessing.

For the other 5% of times that the doctor is late to his appointments due to his own tardiness, it's ALWAYS because he was taking a phone call from another healthcare provider about one of their mutual patients or working on a note from a previous patient encounter that needs to be sent out immediately so the patient can be seen elsewhere. The doctor I work for is there when I get there at 7:30 and there when I leave at 6 and hardly spends 5 minutes per day not doing some type of paperwork for his patients when he's not in the rooms with patients.

Sorry for the long post (a bit of a rant I admit but something that I think is important) but I will finish by saying, yes there are bad, slow, lazy doctors out there and finding good ones is difficult but ultimately you have the power of your dollar and can choose whom you would like to pay for services. View doctors like any other service industry - if your service at a certain restaurant sucks, how often do you go back there? Shop around until you find a good one that best fits your need for waiting time vs. quality of care.

TL;DR my experience is that long wait times are never a result of the doctor being lazy or unprofessional; as a patient, try to be as respectful as possible of your 20 minutes, even if that means scheduling a second appointment or speaking to a nurse on the phone to clear up any questions you weren't able to ask in the initial appointment; seek out good doctors