Grumpy_Mike:
Isn't it an advantage to have smaller resistors for that?
No, the charge storage comes into effect when the transistor drive is removed, it takes time fro them to be swept out of the transistors base region. The base capacitance will not discharge through the drive resistor because it is a diode.
OK. I thought some of it would go out through the resistor when the Arduino pin goes low.
(I'm just a hacker. I'm not going to argue with somebody who describes himself as "grumpy" and knows all about transistor base capacitance...)