As of December 2023, almost all HFT activity is performed with reaction times below 20 ns.

Stefan Schlamp (who's one of the few public writers on HFT worth reading IMO) posted this Eurex data a few weeks back (Dec 2023). Interesting that tick-to-trade median on fast responses have converged to 20ns... which seems insanely fast even for FPGA. Would be curious on any color how people are getting to sub 50ns.

https://www.linkedin.com/pulse/hft-marketdata-share-stefan-schlamp-dijde/?trk=article-ssr-frontend-pulse_little-text-block