diff --git a/crates/ui-egui/src/timeline.rs b/crates/ui-egui/src/timeline.rs
index 53dcf5b47e..d7a421e74f 100644
--- a/crates/ui-egui/src/timeline.rs
+++ b/crates/ui-egui/src/timeline.rs
@@ -406,17 +406,37 @@ pub(crate) fn status_text(name: &str, ty: &str, error: Option<&str>, warning: Op
 /// it isn't built then.
 pub(crate) fn feature_status(app: &SolveApp, id: u64) -> Option<Status> {
     let index = app.session.doc.feature_index(id)?;
+    status_at(app, index, || app.session.model.result(id))
+}
+
+/// The mark of the feature at `index` in the history, given how to find its result.
+fn status_at<'a>(app: &SolveApp, index: usize, result: impl FnOnce() -> Option<&'a solidcraft_engine::doc::FeatureResult>) -> Option<Status> {
     let feature = app.session.doc.features.get(index)?;
     if feature.suppressed || app.session.doc.marker.is_some_and(|marker| index >= marker) {
         return None;
     }
-    let result = app.session.model.result(id)?;
+    let result = result()?;
     if result.error.is_some() { Some(Status::Error) } else { result.warning.as_ref().map(|_| Status::Warning) }
 }
 
 /// The worst mark among `ids`, with the message of the first feature that has it.
 pub(crate) fn worst_status(app: &SolveApp, ids: &[u64]) -> (Option<Status>, Option<String>) {
-    let worst = ids.iter().filter_map(|id| feature_status(app, *id).map(|status| (status, *id))).reduce(|a, b| if b.0 > a.0 { b } else { a });
+    let mark = |status: Option<Status>, id: &u64| status.map(|status| (status, *id));
+    // A long folder finds each feature's place and result in tables built once: a search per
+    // feature made a folded long history cost the square of its length every frame. A short
+    // one searches, which costs less than building the tables.
+    let marks: Vec<(Status, u64)> = if ids.len() < 16 {
+        ids.iter().filter_map(|id| mark(feature_status(app, *id), id)).collect()
+    } else {
+        let places: std::collections::HashMap<u64, usize> = app.session.doc.features.iter().enumerate().rev().map(|(i, f)| (f.id, i)).collect();
+        let results = result_index(&app.session.model.results);
+        let status = |id: &u64| {
+            let index = *places.get(id)?;
+            status_at(app, index, || results.get(id).and_then(|k| app.session.model.results.get(*k)))
+        };
+        ids.iter().filter_map(|id| mark(status(id), id)).collect()
+    };
+    let worst = marks.into_iter().reduce(|a, b| if b.0 > a.0 { b } else { a });
     match worst {
         Some((status, id)) => (Some(status), feature_message(app, id)),
         None => (None, None),
@@ -1873,6 +1893,58 @@ mod tests {
         assert_eq!(index.get(&twice[first].id), Some(&first), "the first result of an id wins, as in Model::result");
     }
 
+    /// A folder's mark is the worst of its features' own, searched for a short folder and found
+    /// in tables for a long one: an error over warnings, the message of the first feature with
+    /// the worst, none for a feature suppressed or rolled back, and a feature listed twice in the
+    /// history found at its first place.
+    #[test]
+    fn a_folders_mark_is_the_worst_of_its_features_own() {
+        use crate::ui_kit::Status;
+        let mut h = two_boxes(false);
+        let third =
+            json!({"length": 10, "width": 10, "height": 10, "corner": [40, 0, 0], "operation": "new_body", "name": "Third", "body_name": "Three"});
+        h.app.run("solid.box", third).unwrap();
+        h.frames(2);
+        let ids: Vec<u64> = h.app.session.doc.features.iter().map(|f| f.id).collect();
+        assert_eq!(ids.len(), 3);
+        let (a, b, c) = (ids[0], ids[1], ids[2]);
+        for (id, error, warning) in [(a, None, Some("a")), (b, Some("b"), None), (c, None, Some("c"))] {
+            let result = h.app.session.model.results.iter_mut().find(|r| r.id == id).unwrap();
+            result.error = error.map(str::to_string);
+            result.warning = warning.map(str::to_string);
+        }
+        let own = |h: &Harness, ids: &[u64]| {
+            let worst = ids.iter().filter_map(|id| super::feature_status(&h.app, *id).map(|s| (s, *id))).reduce(|a, b| if b.0 > a.0 { b } else { a });
+            (worst.map(|w| w.0), worst.and_then(|w| super::feature_message(&h.app, w.1)))
+        };
+        // Every folder short enough to be searched, then the same repeated past the length that
+        // builds the tables.
+        let check = |h: &Harness, folder: &[u64], status: Option<Status>, of: Option<u64>, why: &str| {
+            for ids in [folder.to_vec(), folder.repeat(16)] {
+                let found = super::worst_status(&h.app, &ids);
+                assert_eq!(found, (status, of.and_then(|id| super::feature_message(&h.app, id))), "{why}, {} ids", ids.len());
+                assert_eq!(found, own(h, &ids), "{why}, {} ids", ids.len());
+            }
+        };
+        check(&h, &ids, Some(Status::Error), Some(b), "the error outweighs the warnings");
+        check(&h, &[a, c], Some(Status::Warning), Some(a), "the first warned feature's message");
+        check(&h, &[c, a], Some(Status::Warning), Some(c), "the first in the folder, not in the history");
+        check(&h, &[], None, None, "an empty folder");
+        check(&h, &[u64::MAX], None, None, "an id outside the history");
+
+        let mut twice = h.app.session.doc.features[0].clone();
+        twice.suppressed = true;
+        std::sync::Arc::make_mut(&mut h.app.session.doc).features.push(twice);
+        check(&h, &[a], Some(Status::Warning), Some(a), "a feature listed twice is found at its first place");
+        std::sync::Arc::make_mut(&mut h.app.session.doc).features.pop();
+
+        std::sync::Arc::make_mut(&mut h.app.session.doc).marker = Some(1);
+        check(&h, &ids, Some(Status::Warning), Some(a), "the features after the marker have no mark");
+        std::sync::Arc::make_mut(&mut h.app.session.doc).marker = None;
+        std::sync::Arc::make_mut(&mut h.app.session.doc).features[1].suppressed = true;
+        check(&h, &ids, Some(Status::Warning), Some(a), "a suppressed feature has none");
+    }
+
     #[test]
     fn the_feature_menu_copies_a_warning_message() {
         for left in [false, true] {