I ran into a similar issue where I had long, opaque “task” taking several seconds without further information in the Dev Tools.
A Chrome dev pointed me towards Perfetto (edit: or you can access a built-in similar tool at chrome://tracing). Perfetto lets you record traces of the internals of Chrome. In my case I suspect this was GPU related so I enabled all the gpu* switches:
and started my trace. After I repro’ed the issue in another tab, I went back to the Perfetto trace. I found these “ThreadControllerImpl::RunTask” slices that seemed to be about the duration of the mysterious system tasks.
Helpfully Perfetto has an arrow from that task to another group of “slices”.
And each of these is catagorized as “webaudio”. From that I learned that my use of AudioContext was likely the culprit, and indeed if I disabled all audio on my page the issue goes away.
Hopefully this example is illustrative to others trying to solve opaque “system task” issues in Chrome.