Geeetech

Black plastics is most problematic one.

Try my profile for A10M, propably you need to change build size and zero position .

For PEI sheet delete line from gcode setup
G1 X12 Y5 Z0 F300 ; stop leaking hotend
otherwise your PEI sheet will have burned hole at zero position.

You need to clean hotbed with isopropyle and leave it covered against dust. It must be clean, dark black... PEI sheet is much worse.

I think that it could be mechanical reason - did you tighten your printer?

Mixing sometime does crazy sounds. Try to use rubber or cork dumper between extruder and frame.

No, it is pretty fast.

10mm3/s is not 10mm/s, but 8-30x more of print speed.

Hmm, I look that Geetech made very good changes to original Prusa....

At your problem - I think that it is misaligned rods.

See at Original Prusa crap:

Try it with my profile for A10M (define Filament/Extruder multiplier as 0.95, Print/Speed/Volumetric speed 10, Printer/Extruder/Retract 8mm).

Look at my signature

Filament, Extrusion multiplier. Set value by 0.07 lower. Auto width is not good for all prints. Look at A10M HQ profiles. Low heights are problematic. See my profile for A10M (not A10) where is implemented height temperature profile and profile for low objects. At github is more recent version than ...

Sorry, my mistake, A10 <> A10M... You really need to apply bigger current to extruder stepper motor, but... A10 stepper is easy to overheat if you print with ABS. Measuring of temperature is easy: If you put your hand to extruder and you have no problem to hold it more than 10 seconds, then it is co...

You can use it with original firmware, but adjust only hotend PID values and save to eeprom (use Pronterface to do that - via gcode cun calibration manually). Did not forget to take photo of old values. And then in Slic3r at printer and filament configuration make appropriate changes. And of course ...