Side-effects strike again! Not only does the hardware have observable side-effects that are not cleaned-up, but our browser languages allow pervasive side-effects which are used for exploits (e.g. precision timing).
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.
