Fix 232ms UI hang in call duration timer

[sampepose]

🔗 Issue Links

N/A - Performance improvement identified through profiling

🎯 Goal

Fix a 232ms UI hang that occurs every second during active calls, caused by expensive Swift runtime protocol conformance checking when accessing @Published properties in a timer callback.

📝 Summary

• Eliminated 232ms/second UI hang by caching startedAt value to avoid @Published property access
• Moved duration timer to background queue using DispatchSourceTimer instead of Foundation.Timer
• Removed unnecessary @objc updateDuration() method by calculating duration inline
• Zero impact on functionality - duration updates work exactly the same, just more efficiently

🛠 Implementation

The issue was discovered through Instruments profiling, which showed swift_dynamicCast and protocol conformance checking taking 232ms when the duration timer accessed the @Published var startedAt property.

Root cause: Every second, the timer would access startedAt through the @Published property wrapper, triggering expensive Swift runtime checks for protocol conformances.

Solution:

1. Cache the value: Added _cachedStartedAt to store the date without going through @Published
2. Background timer: Switched from Foundation.Timer (main thread) to DispatchSourceTimer (background queue)
3. Minimal main thread work: Only the final duration update happens on main thread

This approach eliminates both the protocol conformance overhead AND keeps the timer work off the main thread.

🎨 Showcase

Before	After
232ms hang every second during calls	Smooth 60fps during calls
Timer blocks main thread	Timer runs on background queue
@Published access in hot path	Cached value access

🧪 Manual Testing Notes

1. Start a video call
2. Verify the call duration timer updates every second
3. Monitor performance with Instruments - no more periodic hangs
4. Verify timer stops correctly when call ends
5. Test timer behavior during reconnection scenarios

☑️ Contributor Checklist

• I have signed the [Stream CLA](https://docs.google.com/forms/d/e/1FAIpQLScFKsKkAJI7mhCr7K9rEIOpqIDThrWxuvxnwUq2XkHyG154vQ/viewform) (required)
• This change follows zero ⚠️ policy (required)
• This change should receive manual QA
• Changelog is updated with client-facing changes
• New code is covered by unit tests
• Comparison screenshots added for visual changes
• Affected documentation updated (tutorial, CMS)

[ipavlidakis]

Hey @sampepose, thank you for this PR. We are currently in the process of resolving many performance issues (you can take see a sneak peek in the open PRs).

We are now in a state where we have resolved all the hungs and for that reason I'm not going to accept this PR.

I'm going to keep open though as I reminder to check again once the changes have been merged.

Thanks,
Ilias

[sampepose]

Great, I see this was addressed in #825!