Fine tuning the pen
If the pen lifts too slowly, you might get excessive bleed into the paper. If the pen is not fully lifted before the pen does a move, you might get a little streak.
Test the pen on the paper, some pens flow too much for the paper. If you are getting bleeding, consider a better pen.
The midTbot uses a servo for the Z. Unlike stepper motors, Grbl does not directly control the acceleration and speed of the servo. It tells the servo where to go and the servo goes as fast as it can to get there. Internally, it uses a virtual Z axis with a speed and acceleration. It tells the servo about 20 times a second where to go. Therefore, in theory, you can control the speed and acceleration of the servo. The servo tries its best to track that virtual stepper motor.
This assumes the servo can move as fast as the virtual axis. If it cannot, it lags behind a bit. The speed and acceleration of the Z is set pretty conservative so most servos can keep up.
The servo has a range associated with it in the cpu_map. The default is 5mm. Make sure you gcode uses this value. If your gcode sends it to 10mm, the servo will track it for the first 5mm and then appear to wait as the virtual Z finishes to the 10mm.
You can play around with the $110 (speed) and $120 (acceleration) settings. Try increasing those numbers to see if the servo moves any faster. If you get to a point where is stays roughly the same speed, don't increase it.
If the the servo has to travel a bit before it actually starts lifting the pen, that can cause bleed. How far you place the pen into the holder also has impact on this. There is information on calibrating here.