See bug 1632918. We fixed that bug by moving the first app update check from the first time the urlbar is used to the first time the update intervention is triggered. There's still jank then that we should fix somehow. My comment 5 there discusses some options. In summary, we could make all the IO in the app updater async, or we could just not trigger update checks from the interventions provider at all. That's only two options, there may be others I didn't think of.

Because this bug's Severity has not been changed from the default since it was filed, and it's Priority is P3 (Backlog,) indicating it has been triaged, the bug's Severity is being updated to S3 (normal.)