> Also note you can get precise JavaScript measurements (and threading) by adding some headers

Though you can access these techniques now, in the weeks after Spectre attacks were discovered, the browsers all consolidated on "make timing less accurate across the board" as an immediate-term fix[1]. All browsers now give automatic access to imprecise timing by default, but have some technique to opt-in for near-precise timing.

Similarly, Swift has SuspendingClock and ContinuousClock, which you can use without informing Apple. Meanwhile mach_absolute_time & similarly precise timing methods require developers to disclose the reasons for its use before Apple will approve your app on the store[2].

[1] https://blog.mozilla.org/security/2018/01/03/mitigations-lan...

[2] https://developer.apple.com/documentation/kernel/1462446-mac...