I met people in the Commodore Hawaii User Group (CHUG) many decades ago when I handed over to them my collection of Commodore VIC-20 and C64 computers, peripherals, etc.
One member was writing a realtime stock trading system on his C64 in 6510 Assembler. I have no idea how it turned out. Since the C64 could only do 300baud serial, I doubt it was very close to realtime!