The fans run at low speed due to refrigerant pressure switch. The pressure usually gets to the fan cut-in point pretty quickly when its hot out.
If you are low on refrigerant the pressure will stay low even when hot. The proper answer here is to check the switch pressure and verify with A/C gauges for full charge and cut-in pressure.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician