High-frequency trading, I'd assume. Exploiting the stock market for profit.

A huge black hole of mental energy spent slicing pennies and seconds, creating nothing.

Or providing liquidity to reduce transaction costs for everyone.

How high are the costs and how much can you reduce them before you reach diminishing returns? Will it reach zero?

It is pretty close to zero. Has anyone thought about how much a trade of a broadly traded security will cost them in recent times, or ever thought it would not happen near instantly?

How frequently do you think most people make such transactions?

I don’t know about individuals, but there are quite a few people exposed to transactions costs via 401k/IRA/taxpayer funded pensions/etc.

Even making deposits into the account incurs transactions. Sure, speed doesn't matter here, but overhead does.