Smooth long lists

Performance Mid-level Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Pick the fix

Each feed row calls items.sortedBy { it.date } and formats dates inside the composable. What's the best fix?

OptionsSort and format once in the ViewModel, and pass ready UI models to rowsWrap each row in rememberUse a bigger thread poolDisable animations

Hint 1 of 1

How often does this work run?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A Sort and format once in the ViewModel, and pass ready UI models to rows

Correct. Work happens once per data change, not per recomposition.

Why not the others?

B Wrap each row in remember. Rows still recompute when their inputs change, and it doesn't move the sort.
C Use a bigger thread pool. Composition runs on the main thread.
D Disable animations. Unrelated.

30-second answer

Give items stable keys and content types so the list reuses compositions, keep item composables cheap (no sorting or formatting during composition), load images at the displayed size with a caching loader, avoid reading fast-changing state in item bodies, and precompute UI models in the ViewModel. Then measure with a release build and FrameTimingMetric.

Practise saying it · 35 s. No recording, just a timer

Explain how to make long lists smooth.

Say your answer out loud, then check which of these points you covered:

Keys and content types
Cheap item composables
Right-size images
Precompute in the ViewModel
Measure release builds

Deep dive

In Compose, prefetch is built in; nested scrolling lists need fixed sizes or LazyColumn items, not nested lazy lists in the same direction.

Formatting dates and prices in the ViewModel avoids per-frame allocation.

Debug builds of Compose are much slower; never judge scrolling there.

In RecyclerView, use DiffUtil and avoid notifyDataSetChanged.

Common mistakes

Sorting or filtering inside the composable.
Loading full-size images into thumbnails.
Judging scroll performance on a debug build.

Senior take & follow-ups

Give a checklist in priority order, and say how you'd measure.

Follow-up questions

Think of your answer first, then open the hint.

What does contentType do?
Show hint
Lets the lazy list reuse compositions between items of the same type.
Where should formatting happen?
Show hint
In the ViewModel or mapper, once per data change.

Compare on iOS: Smooth scrolling in lists The same checklist, with cell reuse instead of compositions.

Suggest a fix if something here is wrong or out of date.