Why would a GFCI receptacle causes my electric tools to run slow?
It sounds as though the voltage at the appliance is reduced when the motor is running. This happens when the wire length exceeds the maximum limit for the expected current. Said another way, the voltage drop on the line between the breaker and the GFCI is too great for that amount of current. This suggests the wire size is too small (e.g., 12 ga vs 10 ga). If a light load (100W bulb, radio, etc.) is plugged in, does it work as expected?