There are 2 broad areas. On the fast path, you need to make the decision to trade, check compliance, credit etc and execute the trade. Not just quickly but qui

There are 2 broad areas. On the fast path, you need to make the decision to trade, check compliance, credit etc and execute the trade. Not just quickly but quicker than everyone else. This translates into a low latency problem, as others have said. But unlike many real time engineering problems, the environment is actively working against you so it's a perpetual race to the bottom. To the point that the speed of light is annoyingly sluggish. The other big area is trading strategy. The majority of the work here is done offline and is more about analysing a lot of tick data than latency. Big data tooling is pretty common here and machine learning is getting more prevalent.