I thought it might be worth a quick note on this, in case anyone finds the information useful: the RGB LED on the Nano RP2040 Connect is very slow. A call
digitalWrite(LEDB, HIGH);
takes 0.26 milliseconds (yes, milliseconds, not microseconds). I know it's physically connected to the NINA, so switching it involves some kind of communication to that device, but a carrier pigeon would be quicker
I spent a significant part of yesterday working out why my code was not keeping up with incoming data. Guess what: I was using the RGB LED to indicate data overrun, and that was causing the problem in the first place!