The most important factor is how well can your language be optimized. HFT is about winner takes all. If the rust optimizer is even slightly worse than your competitors language you will make nothing. Development speed might get your a faster algorithm for a few weeks but your competition will notice you making that money and will catch up despite the slower pace of development and then their faster language will make the difference and you lose all future trades.
llvm is not known as the best optimizer though. (but benchmarks tend to lie and llvm is always pretty close). There are also subtle areas where the front end can generate code that the backend cannot optimizer as well (though given equal effort I'd expect this advantage to go to newer languages that are design for modern optimizers - but effort is not equal with C++ getting for more love)
Yes, my point is mostly informative; we get benefits from using their optimizer, and all the time and resources others pour in, so it's not solely about what the Rust team does. Your point about front-end optimizations is true though; we generally try to copy what clang does, but there's stuff we can do better as well. And also implementing some of our own front-end optimizations.