It’s the same with all Gelbasters or even Airsoft.
Constant single fire means that the battery, trigger, motor, wiring, electronics etc is constantly having to apply full power through the system from a dead stop in rapid succession, which requires huge current draw to operate.
Starting and stopping the system takes much more strain than just holding the trigger on full auto allowing the motor and gearbox to run at free speed under no load.
Repeated single fire can be felt through the heat generated in the pistol grip, which is a good warning that you should really back off between single shots or switch to burst/full auto if the rules allow to give time for everything to cool down 