IEX claims that their delay means that large orders are able to be carried out without the price changing. If this is true, it would seem that IEX is not just a marketing gimmick and is in fact doing what they say they do.
If this is not true, I don't see why anyone would care about IEX at all, and why they would trade on it.
Yes, this claim is untrue. Large orders will move the market the way they always move the market and 350 micro delay won't change it one bit.
they offer lower fees than other exchanges - standard practice to steal away business from competitors. Traders will trade anywhere where there is liquidity.
I'm willing to believe you but do you have any source at all? Are you saying big block traders are staying on IEX because the fees are lower? That just doesn't make sense to me, the fee overhead is almost insignificant with big block trades. It's not a question of do the trades move the market, it's a question of can the trades fully execute before the market has moved. Obviously they move the market.
I just want to make sure I'm understanding you properly: you are saying that big block traders are using IEX because it has low fees, not because the IEX system is preventing the pricing from shifting during the order, correct?
yes. Also "big block traders using IEX" is a big statement. IEX has 1.5% of US equity volume and at least 20% of IEX volume are not big block traders. So, they aren't really using IEX that much. But to the extent they are using it, it's because the fees are lower and also it's because it's always nice to put pressure on other exchanges via additional competition for liquidity
If this is not true, I don't see why anyone would care about IEX at all, and why they would trade on it.