I run fade up and down code and it looks "fast enough" for me, no lag or blinking. But if circuit is bad I want to know how to do it right.
It is the signal that is slow not anything you do with it. What you might see is that the numbers you put in do not change the brightness as you would expect. You will also notice that the FETs get hotter than they should do.
The way to do it correctly is to invert the signal feeding into the FET, or to have the circuit you have but use a P-channel FET in the +12 line with the + of the strip connected to the FET's source and the -ve of the strip to the ground.