I'd like to disagree. The language specifications provide various limits on the kind of performance you can get, regardless of the platform.
Performancewise, for the record, I find JavaScript to be wholly adequate for writing servers, but to say that the platform is what makes it slow(er) doesn't ring true to me. For example, JavaScript has a few poor design decisions that make implementing performant arrays really hard. Some other interpreted languages get this right, and their performance can be (and usually is) much better in that regard. However, this is kind of splitting hairs since you only care about its performance on whatever environment on which you are running it.
Performancewise, for the record, I find JavaScript to be wholly adequate for writing servers, but to say that the platform is what makes it slow(er) doesn't ring true to me. For example, JavaScript has a few poor design decisions that make implementing performant arrays really hard. Some other interpreted languages get this right, and their performance can be (and usually is) much better in that regard. However, this is kind of splitting hairs since you only care about its performance on whatever environment on which you are running it.