It's worth noting that even "mature" languages are often not fast enough. I wrote several of the optimized encoders and decoders used by the compression library on Apple platforms; the core algorithms are mostly in assembly because it's often possible to go 20-200% faster than C or C++ for that type of workload.
The optimization tasks necessary to get that speed in Swift are awesome to work on, but there's years of work to be done to get there. In the meantime, it probably makes sense to wrap Swifty APIs around the fastest, safest implementations we have available.