perf(engine): look up a component's asset root once per usage report, and skip mode migration without instances (#1966)

crates/engine/src/assets/components.rs

@@ -147,6 +147,10 @@ pub fn inside_instance(doc: &Document, id: NodeId) -> bool {
 /// main layer's, for files from before `modes` was an override group. Copies follow their main
 /// layer's modes, so a layer on other modes chose them itself.
 pub fn mark_layer_modes(doc: &mut Document) {
+    // Without an instance nothing can be inside one: skip the per-layer ancestry lookups.
+    if !doc.nodes().any(|n| matches!(n.kind, NodeKind::Instance { .. })) {
+        return;
+    }
     let chose: Vec<NodeId> = doc
         .nodes()
         .filter(|n| !n.overrides.contains("modes") && n.source.and_then(|s| doc.node(s)).is_some_and(|m| m.var_modes != n.var_modes))

crates/engine/src/assets/library_server.rs

@@ -79,10 +79,13 @@ pub fn usage(doc: &Document, library: &str) -> Usage {
     let records: std::collections::BTreeMap<NodeId, NodeId> =
         doc.libraries.components.iter().filter(|r| r.library == library).map(|r| (r.root, r.key)).collect();
     if !records.is_empty() {
+        // One component's asset root is looked up once, however many instances use it.
+        let mut roots = std::collections::BTreeMap::<NodeId, Option<NodeId>>::new();
         for n in doc.nodes() {
             if let NodeKind::Instance { component, .. } = &n.kind
                 && store.is_none_or(|st| doc.page_of(n.id) != Some(st))
-                && let Some(key) = crate::library::asset_root(doc, *component).and_then(|r| records.get(&r))
+                && let Some(key) =
+                    roots.entry(*component).or_insert_with(|| crate::library::asset_root(doc, *component)).and_then(|r| records.get(&r))
             {
                 let c = counts.entry(*key).or_default();
                 *c = c.saturating_add(1);