From the Spectre paper: Chrome reduced resolution of performance.now(), so they set up a webworker with a tight loop that increments a shared variable.
Yes and creating one or more "web workers" (threads) is a significant side-effect. Javascript to me, looks more like a systems-programming language (imperative with threads and sockets) than something trusted for interactive web sites. The web is a huge mess and today we learn that hardware is too.
