It just runs Julia under the hood, so I would expect it to be simply as fast as Julia (assuming that the data processing is the more significant bottleneck compared to the HTML output that is used to present the results). Performance more likely affected by the way the data is processed than the language's speed.

From what I understand, results of cells are cached though, and they don't update unless something upstream changes, so there is a form of memoization happening. Which of course also has both implications for performance as well as memory usage.