I'd say starting a "pure" HFT firm is incredibly hard nowadays. There's a big upfront investment and huge operating costs, and there's a lot of consolidation happening in the industry.
In general, most hardware and software is tailored to high-throughput, and a huge part of the effort of HFT is fighting all that to prioritize latency. If you want to compete with the very best firms, that means renting rackspace in a colocation with the exchanges, overclocked CPUs, expensive network cards, FPGAs, paying out the nose for the most accurate market-data feeds, and many order-entry connections. Then you have to tweak the BIOS, kernel settings, isolate your processes on cores, write your own userspace networking stack etc. etc. etc. On top of that if you want to trade between different colocations, you might need a network of microwave towers because fiber is too slow (light travels about 2/3 slower in fiber because it effectively "bounces" along the fiber). At the end of the day though, if your algorithm or strategy doesn't work, you won't make any money - speed is only a part of it. At the previous firm, we had a market making desk that was very profitable, but was quite "slow" by HFT standards (triple digit microseconds vs sub-microsecond response times). You still need to be smart about it.
Having said all that, I think HFT gets a bad rap, about being a waste of resources/talent, and somehow "stealing" from the common man. The truth is, they are only "stealing" from each other - they compete to remove the tiniest inefficiencies in pricing. They ensure the cost of trading is actually as low as it can possibly be. By themselves they don't have enough capital to move the market in any meaningful way. In fact, most flash crashes occur when HFT firms step out of the way! They move out of the way of huge market movements because they can't cushion the blow without taking an unnecessary risk as a business. It's like asking your plumber to fix your sink while your house is on fire. Market orders, stop orders and panic are most of the problem when it comes to flash crashes. PSA: Please use limit orders :)
I might ruffle some feathers with the comments, but I think overall HFTs aren't the bogeyman most people think them to be. Sure there are some bad apples, but that's true of any industry, and it comes down to the people - not the system itself.
> At the previous firm, we had a market making desk that was very profitable
Are you saying, it was guaranteed to make money? I.E. the algorithm never failed, or never made losing trades? That seems insane, to always be able to make money even if it's fractions of a percent.
How does "very profitable" equate to "never made losing trades?" If you're only right 60% of the time you can still be profitable overall with proper risk management.
