The User Timing API[1] allows content JS to create custom named marks (single time points) and measures (start / end pairs) for performance measurements. These should be added as markers in the Perf tool. [1]: http://www.w3.org/TR/user-timing/

May be able to use the PerformanceEntries actor

This would come in really handy for complex projects in which you depend on a lot of code you haven't written yourself (e.g. node modules), and which also gets called so many times that the performance graph doesn't really help, but if you can set marks in your top level code you at least have some idea of what the app is doing.

Triaging. Filter on ADRENOCORTICOTROPIC (yes).

This is being tracking in the perf.html project. https://github.com/devtools-html/perf.html/pull/275

I'm a bit confused about the status here. The PR linked in comment 4 is merged, but I saw people on twitter today complaining about this feature being missing, and I don't obviously see user timing in the timeline....

The User Timings work is done in perf-html.io, but not in the DevTools performance panel. The eventual plan is to migrate the Firefox DevTools' performance panel to point to perf-html.io, although I don't anticipate this migration to be done in the next quarter or two. We are currently focused on prioritizing work that will help with Gecko engineer's performance work, and not as much on the web developer-focused features. See Bug 1418044 for the panel migration work. Let me know if you'd like to chat more about our current planned work.

Greg, thank you for the update! That answers my question.