There are a number of issues you'd need to overcome with speed. First, the motor stepping frequency causing electromagentic noise and loss within the motor cables. Since they're not signal wires, they carry the full power needed to drive the motors. So when they quickly alternate back and forth, between different poles, switching polarity, etc., it's quite an EMI crapfest that causes the wires to negate each others' signals at higher speeds. So you lose steps (slipping easier), lose torque, and sometimes the motor just grinds if it goes too fast. The default firmware settings essentially have no limits - 3000mm/s (as I believe I've seen default) is about 6.5 MPH, so let's picture your printhead moving at the speed of a bicycle for a split second.
The problem is that the firmware settings are tied to "speed" instead of "steps per second" as they should be. Speed could physically change, if the motors have different gearing... but all the X/Y/Z/E motors have the same rotational-speed limitations. Seems outright foolish that Marlin would tie the limits to axis speed instead of motor rotation speed.
Second, the extruder can't keep up with high speeds of anything. So if you're trying to blaze a quick straight line of plastic, you can forget it! The limit you can get is when you raise Z and disable motors, heat up the extruder, then see how fast you can turn the gear by hand without stripping. That's how fast the thing can ever print - and even then, it's a crapshoot that the faster you print, the less likely the layers will be to stick to each other, as the extruded plastic won't have enough heat stored to be able to melt and fuse to the underlying layer.
Third, the biggest problem with the Printrbot design, is X/Y jerk. When you go faster, it tries to apply that to everything. So when you go around the teeth of a motor or some narrow infill, the machine snaps back and forth on each segment, vibrating the machine and causing belts to slip, motors to skip, etc. Inertia is a bitch. I found some ways to minimize that jerk by having it slow down (further) around small segments: viewtopic.php?f=28&t=1116
- but you really can't (shouldn't) do much about the small segments.
Slower = better for most cases. Much as I'd hate to say... otherwise you'll probably just end up crapping out junk prints.