Speaking only for R, most of the performance-critical functions included in packages these days are just wrappers to a C++ implementation. It isn't until you need to implement something custom that performance starts to be an issue.
That combined with it's ease-of-use working with tabular data, and the myriad of packages available make it a very popular choice.
If Rcpp were not a thing, R might have died from being slow. Since it is a thing, though, people can easily write packages that are certainly fast enough for what they want to do, and that keeps the language useful.
That combined with it's ease-of-use working with tabular data, and the myriad of packages available make it a very popular choice.