At home I have a power supply with 12V and 30A. Can I use it lowering the voltage with a resistor or the high current will burn the servos?
No you cannot reduce the voltage with a resistor.
The reason is the voltage drop across a resistor will change when you change the load so....
If you pick a resistor so the servo received 6 volts ( at a very low current) then when you use the servo to drive something the load will increase and the voltage across the resistor will increase. The servo will receive only a few volts.
If you pick a resistor so the servo received 6 volts (at full load) then when the servo has no load the voltage to will be near 12 V, causing your servo to burn out.
Thanks to you all!
Now I have to study how a voltage regulator circuit is made and how to build it, but at least I know where to start.
You've missed the point. Again.
With the servo.write() in setup() it is only called once and the servo library then produces a continuous 50 Hz stream. That is not and never has been a problem which is why I suggested moving the write out of loop() into setup().
If you put the write() in loop() on its own it gets called very frequently. There are circumstances where calling write() again before the previous write() is fully established can cause glitching in the signal (and no I can't follow the code well enough to work out exactly when). Yes it still operates at 50Hz but the pulse length is not steady.
If you want to test that it might be interesting but your current tests prove absolutely nothing.
Steve