A janky animation = the browser can't finish a frame within its budget (~16.7ms at 60fps), so frames drop. The approach is measure → diagnose → fix → verify — never guess.

Step 1 — Reproduce and measure

• Test on a real mid-range device or use DevTools CPU throttling (4–6×) and network throttling — desktop hides mobile jank.
• Open DevTools → Performance, record the animation.
• Look at the frames track: long frames (>16ms) are jank. Inspect them:
• Purple bars = Layout (reflow) — you're animating a layout property.
• Green bars = Paint — large or expensive paint areas.
• Yellow = scripting — JS is the bottleneck.
• The Rendering tab: enable "Paint flashing" (see what repaints) and "Layer borders."

Step 2 — Common causes of mobile jank

1. Animating layout/paint properties — width, height, top, left, margin, box-shadow. Each frame triggers reflow/repaint on the main thread.
2. Large paint areas — animating something that forces a big region (or the whole page) to repaint.
3. Too many / too few compositor layers — none means no GPU offload; too many exhausts mobile GPU memory.
4. JS-driven animation on the main thread, blocked by other work.
5. Heavy effects — big blur(), complex box-shadow, many gradients — GPUs on phones are weak.
6. Layout thrashing — reading layout (offsetHeight) inside the animation loop.

Step 3 — Fixes

• Animate transform and opacity only — compositor-only, GPU, off-main-thread. Replace left with translateX, width with scale.
• Promote the element: will-change: transform (or transform: translateZ(0)) — sparingly, remove after.
• Shrink the paint area — animate a small element, not a full-width container; use contain: paint.
• Move JS animation off the main thread — use CSS transitions/animations or the Web Animations API instead of setInterval; or use a Web Worker for the computation.
• Reduce effect cost — smaller blur radius, simpler shadows, fewer animated nodes.
• Use requestAnimationFrame for any JS-driven animation, never setTimeout.
• Respect prefers-reduced-motion — disable or simplify for users who opt out (also an accessibility win).

Step 4 — Verify

Re-record. Frames should be green and under 16ms. Check on the actual device, not just throttled desktop.

Senior framing

The senior answer leads with "profile, don't guess" — name the DevTools workflow and what the colored bars mean. Then the fix is the rendering-pipeline insight: jank = reaching Layout/Paint per frame, so move to compositor-only properties. Mentioning mobile-specific realities (weak GPUs, layer-memory limits, CPU throttling to simulate) and prefers-reduced-motion shows you've actually debugged this, not just read about transform.