C# syntax is faster than Java because Java has no way to define custom value types/structs (last time I checked, I know there was some experimental work on this)

and yet there's more Java in HFT than C#

And don't get me wrong, I'm C# fanboi that'd never touch Java, but JVM itself is impressive as hell,

so even despite not having (yet) value types/structs, Java is still very strong due to JVM (the implementation). Valhalla should push it even further.

It may also be related to the fact that .NET until now does not have any GC with super low latency pauses, but there are interesting development going on recently regarding this - https://github.com/dotnet/runtime/discussions/115627.

HFT code is unusual in the way it is used. A lot of work goes into avoiding the Garbage Collection and other JVM overheads.