How much of that is genuinely down to HFT, and how much is down to changing the "protocol"? Keep in mind that in the bad old days, it was simply impossible to p

How much of that is genuinely down to HFT, and how much is down to changing the "protocol"? Keep in mind that in the bad old days, it was simply impossible to place orders at a cent-level granularity for most stocks. You could probably get the same benefits by (a) forcing exchanges to allow something like a 10^-9 granularity of prices, (b) restricting matching engines to operate on something like a 15 minute heartbeat, (c) regulating the crap out of exchanges to lower the barrier of entry. You'd get reasonably low spreads while eliminating the waste of resources caused by the high speed arms race. At the same time, for individuals who actually invest (rather than speculate/trade), the spread is a distraction. Access to markets and fees are the real hurdle.