All stepper drivers have some resistors, sinking the current from motor coils into ground. Because the coil current passes through them, then a voltage appears across the sense resistors, which the driver takes as input and this is how the drivers knows exactly how much current is in the coil. Hence the name of sense resistors. There are two of them, one for each coil, and have very small values. The a4988 datasheet mentions I_TripMAX = V REF / ( 8 * Rs), and if Rs=0.05ohms, that gives some result, which differs for example if Rs is 0.10 instead. About pfd sry i was thinking of something else, for this driver can only play with Rosc and on the driver boards you can try short across Rosc resistor to ground and see if you get an improvement - on the pololu site schematic that is labeled R4, and also from that schematic image i think check that R2 does not exist (not populated) because only either one of the resistors should be there.
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