that nearly every publicly traded stock feels real-time.

I’m in commodities, but I think this applies to stocks too. There are professional “market makers” who are constantly buying and selling the popular stocks to keep the markets liquid. They can make a commission off these trades or create technical moves to offset their trades, like upticks in a downward sell-off.

Along with these computers making thousands of transactions you have limit orders, options, strikes, etc. all pricing along the way. There’s just a crapload of activity everyday.