Pico MAX7219 C
The MAX7219 chip uses a simple serial protocol at any speed up to 10MHz. This C code implements the protocol for the Raspberry Pico. It can update an 8-character display at several kHz. Rewriting it to use the Pico’s PIO should make it much faster - the MAX7219 should be able to update at 62 KHz.
Of course, this is far faster than human perception, but offloading the bit-banging to the PIO would let the processor handle real-time tasks better.
Any number of MAX7219s can be chained - bits just get shifted from one to the next. All chips will execute their commands simultaneously when the LOAD pin goes high.
The MAX7219 chips I have work fine with 3.3V logic, though the chip nominally requires 5V logic.