It depends on the source language. We have done extensive benchmarking. For example, on Python the median slowdown is 1.4x. PyJS doesn't use all of JavaScript's features and we can exploit that to improve performance. Our worst slowdown is when the source language is JavaScript itself because we can't make simplifying assumptions. (Median slowdown is 20x.)

These are slowdowns on a cheap ChromeBook. We also have results from the four major browsers on a desktop machine.