in configuration.h you will find
// Homing speeds (mm/m)
#define HOMING_FEEDRATE_XY (50*60)
#define HOMING_FEEDRATE_Z (6*60)
lower these values and the probing should be slower.
and
// Feedrate (mm/m) for the "accurate" probe of each point
#define Z_PROBE_SPEED_SLOW (Z_PROBE_SPEED_FAST / 2) <-- The "2" means, that the second probe is half of the first speed. Do you need it even slower, rise that value.