I’m using a Pi 3, 2 64x128 for a total 64 x 256 array, and a 5 volt 3A power supply. When the display first starts out the text runs nicely across the matrix for the first 10 - 15 characters then it slows down for 10 - 15 characters. Sometimes I get a nice burst of characters before it slows down. I’m running with a wireless network adapter and noticed network activity occurs when this happens. Would this be a power issue or network issue?
I’ve found, at least with my Pi3, it’s VERY sensitive to what else is happening on the Pi. When something else spins up (via cron, or over ssh or whatever) the display code gets lower priority and slows down. I’ve had less of an issue with a Pi4, but it’s still my same slow python code either way, so 
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Yeah I’m thinking it’s probably a combination of Pi3 and Python. Guess I’ll have to learn some C++