I may err, but I don't really think it's that big issue. First off, "low level" isn't exactly synonymous to "efficient machine-code", I guess we well can expect

I may err, but I don't really think it's that big issue. First off, "low level" isn't exactly synonymous to "efficient machine-code", I guess we well can expect from languages like Rust to become both efficient and predictable after a while. In fact, every non-lazy well designed static typed language implementation makes it very much possible to write code that compiles in pretty predictable way, so you can control execution time as well. And the last, but not the least, timing attacks aren't chief vulnerability, really. If you expect a timing attack being real hazard you can randomize return time explicitly.