I’ve think you’re using an Arduino Due, it has USB serial. One small problem in your program is calling thread(“requestData”) at every draw(). With frameRate(120) this is not practical. I suggest you call this only every second or so.
Not sure if this is a problem, you’re transmitting on USBSerial at 9600, much slower than receiving on Serial, but your not transimitting as much. There has to be time for each transmission channel to keep up with what’s expected. Another confusion for me is the baud rate discussion here.
I have your sketch in and Arduino Mega, and tomorrow I intend to connect a ttl-usb-serial module to Serial2 so I can see what you are printing on USBSerial.