Let me engage my traffic engineer brain for a moment and think about multi-modal travel time from All-Star Music to the Crystal Palace for an ADR under three different scenarios to illustrate the need for reasonable planning. FYI, none of these involve waiting 40 minutes for a bus, which for argument's sake we'll assume is a low-percentage occurrence among the hundreds of stops buses make around WDW every day. And for heaven's sake, this illustration is for fun!
On a Perfect Day
The bus shows up the second you walk up to the All-Star Music stop. The bus takes you directly to MK, with no stops at other resorts. From All-Star Music to the MK bus stops is 6 miles, which at 35 mph is about 10.5 minutes, with no delays/red lights. From the MK bus stop to security is 1000ft, which at a comfortable walk is about 3.5 minutes. You have no bag check, and there is no line at the metal detector (woo-hoo!), so you cruise right through turnstiles with no wait, under the train station, and onto Main Street USA 90 seconds later. The Crystal Palace is another 900ft walk away, and you're there in just over 3 minutes, all while singing birds land on your shoulder and CMs applaud your arrival. Perfect Day time is about 19 minutes.
On a Reasonably Good Day
You show up to the All-Star Music stop 10 minutes after a bus just left, and another bus shows up 10 minutes later. The bus is on a loop and makes a couple of other stops, so throw in 5 minutes (assuming no nightmarish CBR scenarios). The trip to MK doesn't have any traffic jams, but there are a couple of red lights at the intersections in between, say 12 minutes. Getting off the bus with strollers, getting set up, and moving to MK security, say 5 minutes. There is a reasonable line at security, and you are carrying bags, how about 5 minutes? That delay carries through to getting past the turnstiles, another 5 minutes. Walking to Crystal Palace through a moderate crowd is about 5 minutes, and you hear a bird chirp somewhere above you as a CM recommends you to have a magical day. Reasonable Day time is about 47 minutes. That 45-minute expectation is doable, but no room for any unexpected day.
"You Were Bad in a Previous Life" Day
You watch the tail-lights of the bus pull away as you reach the resort stop, and another arrives 20 minutes later. Buses are crowded and full of strollers, so the other stops on the loop take a bit longer, 10 minutes. There is a moderate traffic jam on World Drive, 20 minutes. Walk from bus to security with strollers, still 5 minutes. Security bag/stroller check is a longer line, 10 minutes. Line through the turnstiles, 10 minutes. Slogging through level-10 crowds to Crystal Palace is 8 minutes, and the birds are silent as a pop-up thunderstorm sends CMs ducking for cover. Bad Karma time is about 83 minutes.
Now, you could rightfully dispute any of my travel time assumptions, but what I'm trying to impress upon is more about orders of magnitude among these three scenarios. My family loves staying at WDW resorts, and we use the buses and boats every time we stay, but if we are on one side of WDW and need to get to the other... yeah, we're leaving ourselves 80 to 90 minutes to do so, just in case.