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.

This is why Mozilla also removed support for shared variables (i.e. SharedArrayBuffer).