Slow depends on the use case. If your main bottlenecks are in code that people have written highly optimized lower-level libraries for, it could be slower to roll your own implementation or use a less mature library in a different language. And in some cases it might not matter if you take 1 ms or 100 ms to do something.

It works the other way too, and you have to weigh the pros and cons of your specific case when you choose a language. What level of abstraction do you want to work at, how stable do you need the toolchain to be, how familiar you are with the language, what's supported in the language, how easy is it to do what you want, what's the norm in the domain you're working in, etc.