Marlin acceleration issue on corners
It seems that Marlin adjusts acceleration just for print speed, without adjusting it too for the extruder, so the extruder extrudes a constant amount of filament, which on slow speeds (corners) results in an overextrusion and on high speeds (lines) results in an underextrusion, causing bad corners and inaccurate prints.


This is not the case. Marlin accelerates the extruder perfectly in sync with the printhead, and the speed of the extruder is perfectly proportional to the movement speed. However, the extruder itself has a certain amount of "lag": as you feed in filament, pressure builds up. As you stop feeding filament (or slow down), the built up pressure causes the extrusion to continue for a while. Thus, even with "correct" control of the extruder, you don't necessarily get the desired amount of extrusion.

Marlin has a feature, that you can enable in the advanced configuration file, called "extruder advance". It looks at the acceleration/deceleration of the extruder, and advances the extruder a few extra steps when the speed increases, and backs it off again as the speed decreases. You will need to tune this feature to get rid of the overextrusion at the corners.