Below is the information extracted from Futaba 4PL user manual, see whether can help :-)
"When the RX type is S-FHSS(SFH), the servo response can be selected from high
speed (HIGH) and normal (NORM) mode. The high speed mode is exclusively
for our digital servos (including BLS system brushless servo). When using other
servos, select the normal mode. All servos, including digital servos, can be used in
the normal mode. "