Performance metrics, identify bottleneck for checks in individual HTML documents

There are content documents in some publications for which Axe takes several minutes to complete. This has been the cause of timeout issues with the Electron-based Axe "runner" (the CLI Puppeteer-based Axe "runner" does not seem to implement a timeout strategy, but hangs for an equally long time, as the Chromium performance is roughly the same as with the Electron runner).
It would be good to know what part of the Axe process takes so long, or whether this is due to some Ace code. For example, it may well be the HTML outline builder (which is in fact an "at risk" feature that might be removed in a future build, coincidentally:
https://github.com/daisy/ace/issues/269 )