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The democratization of the CME has largely been a good thing and was pushed for by HFT firms. Nobody likes spending time, effort and money optimizing ilink connections. Now, also, it's easier for small teams starting from scratch to get a fair shot speed wise. Coupled with the new corporate membership rental option, I've seen several small groups get up and running profitably recently.
Larger (head count) HFT firms still find pretty lucrative niches outside the US.
Largely echo the sentiment here. The leading HFT firms today are far larger than more profitable than I could have even imagined those years ago. But the diversification into different asset classes (both inside and outside of US) have largely happened, i.e., XTX and the FX market.
I actually see more and more IBanks (I can name 3-4 off top of my head) effectively giving up on market making, ceding the liquidity providing business to HFT firms. We are not far from seeing HFT firms with $1B+ in annual revenue (KCG excluded), Optiver just reported 800M+ / yr rev just a month or two ago. I have a personal "rule of thumb" of approx. $1.5M / per head, that I use to measure success of any HFT firm.
Reason there is very little discussion about HFT on this forum is because there are very few people who knows what they are talking about. A lot of "newbies" asking very basic questions that eventually you get tired of answering and getting no useful information in return.