Build, layout, paint, composite, rasterize: every Flutter frame runs this gauntlet, but most performance advice only covers the first half. DevTools shows your widgets rebuilding. It doesn't show what happens once your layer tree crosses to the raster thread and becomes real GPU work. In the era of liquid glass, that's where the cost hides: backdrop blurs, saveLayer, custom shaders, overdraw. Your phone warms up and you don't know why. (Did you know a spinning progress indicator can keep the whole pipeline awake every vsync and overheat your app?)

We follow one slow frame from build to burn: separate the UI thread from the raster thread, drop into Xcode's Metal frame capture to see the real GPU draw calls, then cover concrete ways to cut that cost. The pipeline and the fixes are platform agnostic; we demo on iOS and explain how the same investigation maps to Android. We close with an AI-assisted Claude Code skill that reads a trace and explains it.