Also note you can get precise JavaScript measurements (and threading, eg. using pthreads and Emscripten) by adding some headers: https://developer.mozilla.org/en-US/docs/Web/API/Window/cros...
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...
Spectre mitigations also are really silly here because as a swift app you already have full access to all in-process memory. It would have to be about meltdown but meltdown is prevented through other techniques.