A feed screen felt heavy while scrolling, only on mid-range phones. Layout Inspector told the whole story: every visible row was recomposing on every scroll frame, including rows whose data had not changed at all.
There was a ghost in the recomposition, and it had one cause.
Compose can skip, if you let it
Compose has a quiet superpower called skipping. When a composable's inputs have not changed, Compose skips re-running it entirely. That is most of why it is fast.
Skipping depends on stability. To skip a composable, Compose has to prove its parameters did not change. Some types make that proof easy. Others make it impossible.
A plain List is one of the impossible ones. List is an interface. The thing behind it could be a mutable list that changed under Compose's feet, and the compiler cannot rule that out. So Compose treats every List parameter as unstable, assumes it might have changed, and recomposes the rows that read it. On a scrolling feed, that means redrawing everything you can see, constantly.
The fix is the type
// before: List is unstable, rows cannot skip
@Composable fun Feed(items: List<Post>) { ... }
// after: an immutable list is stable, skipping works
@Composable fun Feed(items: ImmutableList<Post>) { ... }
Use kotlinx.collections.immutable and build the list with persistentListOf(...) or .toImmutableList(). Now Compose knows the collection cannot change, and it skips rows whose data is unchanged.
While you are there, give the items a key:
LazyColumn {
items(items, key = { it.id }) { post -> PostRow(post) }
}
Without keys, Compose tracks rows by position. Insert one at the top and every row below shifts identity, so more of them redraw than needed. A stable key ties a row to its data.
See it for yourself
Turn on recomposition counts in Layout Inspector and scroll. With an unstable list, the counter on every row climbs. Switch to an immutable list and add keys, and the counters go still. That stillness is the whole point.
The takeaway
Compose is not slow. It is fast by default. Almost every "Compose is janky" story is really the same story: someone handed a composable a type it could not reason about, so it stopped skipping and started repainting the house.
The Recomposer only redraws the room when you give it a reason. Stop giving it reasons.
🌀 Iteration 4 of **The Loopdown. field notes from an engineer who writes.
Series: **Ghosts In The Recomposition* · Featuring: The Recomposer*
📚 The full series: Ghosts In The Recomposition
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