The Boston Marathon course looks like it should be fast. You start out in the distant suburb of Hopkinton—elevation 490 feet above sea level—and then cruise steadily downhill until about mile 9. The finish line has an elevation of a mere 10 feet above Boston Harbor. Fans pack the sides cheering you on. The route is pretty straight, west to east, with few 90-degree turns of the sort that slow your momentum. The road is asphalt, which is more forgiving than concrete.

So when the gun goes off Monday morning for the 123rd running of the race, everyone should feel good about hitting a personal best, right? Of course not. As every veteran marathon runner knows, Boston is slow, wicked, and tempestuous. It’s a wonderful course if you want to experience camaraderie, history, and emotional uplift. It’s a terrible course if you want a personal best.

The average finishing time in Boston is fairly good, but that’s just because you have to qualify for it. People run fast in other marathons so that they can get into Boston, where they will then slow down. The men’s winner in Boston last year ran 2:15, while the winner in Berlin ran 2:01. The 10th place man ran 2:27 in Boston, a time that would have gotten him 72nd in Berlin. The 10th place woman in Boston ran a time that would have gotten her 39th in Berlin.

Of course, one race on one day isn’t a good way to analyze results. The weather last spring on Marathon Monday in Boston was hellacious: driving wind and freezing rain. More than 60 percent of the elite men dropped out. So many elite women dropped out that a Spanish teacher at my Boston-area high school, who had never placed remotely as highly in a major road race before, came in fourth. In other words, 2018 was a particularly bad year to head from Hopkinton to Kenmore Square. But it’s also true that, in general, Boston is slow. And the reason comes down to at least four factors, all of which are partly endemic to the course itself.

The first and most important is temperature. Running quickly generates heat, which the body needs to dissipate. The ideal temperature for running a marathon is roughly 45 degrees Fahrenheit, though the faster you are the colder you want it . Cloud cover is probably good. In Boston, if the sun’s out and you’re not paying attention, you’ll get a sunburn on the right side of your body. Rain, which makes roads slippery and clothes heavy, can feel good but is actually bad. Humidity matters too, since the more vapor there is in the air, the harder it is for a runner to dissipate heat. Boston’s weather in the spring is famously unpredictable. In 2004, temperatures on the course reached 85 degrees. In 2007, the wind chill was in the 20s. In 2012, it went back up into the 80s and more than two thousand runners needed medical treatment for heat-related illnesses.

What’s the weather going to be on Monday? Not great. Enter the current nasty forecast for Boston—15 mph winds, 50 degrees, and 75 percent humidity—into a race calculator, and it suggests one should revise one’s goals downward by roughly two to three minutes. And that isn’t even taking into account the hills.

The second factor affecting the speed of a course is elevation, and downhill is good. But downhill isn’t unambiguously good. Runners have to fire their quadriceps muscles to keep from tipping over when a hill is too steep. The wear and tear builds up over the course of a race, particularly for runners who haven’t been training on similar terrain. And Boston has a series of famously steep uphills between miles 16 and 21, just when your glycogen stores are running out. “Downhill running requires a bit of braking and eccentric contractions that can wipe your legs out,” says Michael Joyner, a former elite marathoner and current sports scientist. “And the hills at Boston are at exactly the right place to make people suffer as a result.”