[jrehor]

It is possible that a block would be sold within 350ms, but how likely is it? That's a function of block size. If you have 100 shares, you'll get it done 100% of the time. 1,000 shares? Somewhat less than 100%. 100,000 shares? Rarely.

These guys are ginormous. $100m block is about a 0.01% position for them. You can't just put a limit order on IEX (or any other exchange for that matter) when you're dealing with this magnitude. You need a team of specialists working full time disguising the order and parceling it out to multiple venues, and it will usually take days to get it done.

The other way of moving a block this size is by finding a natural counterparty and negotiating a deal directly with them. The trade will then get booked through a broker-dealer for reporting and settlement purposes. Those kinds of negotiations take days, too, and you can't avoid showing your hand. Plus, it's not a very scalable solution.