Summary: There are several ways to concatenate strings in Rust. The method listed (using the format! macro) is one of the slowest but also most flexible, it will cheerfully concatenate any things that either are, or present an ability to be expressed as, strings.
The very fastest "method" is to just tell Rust you're quite confident that it will be OK to assume all these strings happen to be contiguous in memory anyway and so it can just go ahead and assemble that memory into one string. Rust won't let you pretend it said this was safe, and sure enough in practice it will almost invariably break, but this is often what equivalent C code is trying for, it genuinely is fast, and if you really are confident you can arrange for the assumption to be true Rust lets you express that in an unsafe block so you get that same performance as C.
The very fastest "method" is to just tell Rust you're quite confident that it will be OK to assume all these strings happen to be contiguous in memory anyway and so it can just go ahead and assemble that memory into one string. Rust won't let you pretend it said this was safe, and sure enough in practice it will almost invariably break, but this is often what equivalent C code is trying for, it genuinely is fast, and if you really are confident you can arrange for the assumption to be true Rust lets you express that in an unsafe block so you get that same performance as C.