A hang is the ugliest failure: no crash, just a frozen UI. The fastest path from “feels stuck” to “this line is blocking the main thread” is a clean Instruments Time Profiler trace and a careful read of the main-thread stack.

• Record on a physical device with Time Profiler and enable “Record Waiting Threads.” Capture long enough to include the stall, then stop and save the trace.
• Verify symbolication before drawing conclusions. Unsymbolicated stacks turn attribution into guesswork.
• Start at the main thread, expand system frames until you hit your code, and look for synchronous I/O, decoding, image work, or cross-queue waits.
• Move heavy work off the main thread and reenter via @MainActor. Align DispatchQoS.QoSClass.userInitiated for UI-visible tasks; use lower QoS for prefetching or background work.
• Add signposts with OSSignposter to bracket decode, image processing, and layout, then re-profile to confirm time moved off the main thread.

If you’ve chased a nasty UI stall recently: what was the actual blocker on your main thread, and what signpost or stack clue cracked it?