The biggest bottleneck in web applications was, is, and ever shall be, roundtrip time to the database, especially now that nobody bothers to install MySQL on the same box anymore, so every query has to go over the network. Of course, that's in a sane web application. Trendier approaches also try to shovel 10 MB of JavaScript over the wire first, so you can then call your API (which is slow for other reasons), and then render the result (which is also slow because you've shoveled 10 MB of JavaScript into the user's browser).

The language you choose matters relatively little.