How do I get XHR/Ajax resource timing data from window.performance?
I figured out my issue after a day of googling and trying different things. Thanks to this article I discovered that I needed to add Timing-Allow-Origin: * to the response header of all the services.
Once I did that, the timing information started to appear. It's apparently because the services are hosted at a different domain than my client. I don't understand the ramifications of leaving that header in there so I'll make sure it doesn't get deployed to production.