diff --git a/crates/engine/src/cmd/insert.rs b/crates/engine/src/cmd/insert.rs
index e4b5bc9d1..83ad977fd 100644
--- a/crates/engine/src/cmd/insert.rs
+++ b/crates/engine/src/cmd/insert.rs
@@ -228,17 +228,24 @@ fn insert_table(s: &mut Session, p: &Json) -> Result<Json> {
         };
         let sh = cx.sheet_mut(sheet)?;
         let mut names: Vec<String> = Vec::new();
+        // Lower-case names taken, and the next suffix for each header, so that thousands of equal
+        // headers don't take cubic time.
+        let mut taken = std::collections::HashSet::new();
+        let mut next_suffix = std::collections::HashMap::new();
         for c in r.start.col..=r.end.col {
             let mut n = sh.value(CellRef::new(r.start.row, c)).display();
             if n.is_empty() {
                 n = format!("Column{}", c - r.start.col + 1);
             }
             let base = n.clone();
-            let mut k = 2;
-            while names.iter().any(|x| x.eq_ignore_ascii_case(&n)) {
+            let key = base.to_ascii_lowercase();
+            let mut k = next_suffix.get(&key).copied().unwrap_or(2);
+            while taken.contains(&n.to_ascii_lowercase()) {
                 n = format!("{base}{k}");
                 k += 1;
             }
+            next_suffix.insert(key, k);
+            taken.insert(n.to_ascii_lowercase());
             // Headers are text.
             sh.set_value(CellRef::new(r.start.row, c), gridcraft_core::Value::text(n.as_str()));
             names.push(n);
diff --git a/crates/engine/src/cmd/pivot.rs b/crates/engine/src/cmd/pivot.rs
index 8dd4b327d..4f9dae595 100644
--- a/crates/engine/src/cmd/pivot.rs
+++ b/crates/engine/src/cmd/pivot.rs
@@ -1621,6 +1621,32 @@ mod tests {
         assert_eq!(val(&s, &CellRef::new(gt, 1).a1()), Value::Number(72.0));
     }
 
+    #[test]
+    fn repeated_headers_get_numbered_names() {
+        let mut s = Session::new();
+        s.new_workbook();
+        let mut header = vec![json!("a"), json!("A"), json!("a"), json!("a2")];
+        // Thousands of equal headers (a row of check boxes, say) used to take cubic time.
+        header.extend((0..3000).map(|_| json!("x")));
+        let data: Vec<_> = (0..header.len()).map(|i| json!(i)).collect();
+        let end = CellRef::new(1, header.len() as u32 - 1).a1();
+        s.execute("range.setValues", json!({"range": "A1", "values": [header, data]})).unwrap();
+        let t = std::time::Instant::now();
+        let src = crate::pivot::read_source(&s.doc().unwrap().wb, &format!("Sheet1!A1:{end}"), 0).unwrap();
+        assert!(t.elapsed().as_secs_f64() < 2.0, "took {:?}", t.elapsed());
+        assert_eq!(&src.headers[..4], ["a", "A2", "a3", "a22"]);
+        assert_eq!(src.headers[4], "x");
+        assert_eq!(src.headers.last().map(String::as_str), Some("x3000"));
+        // Table column names follow the same rule.
+        let t = std::time::Instant::now();
+        s.execute("insert.table", json!({"range": format!("A1:{end}"), "header": true})).unwrap();
+        assert!(t.elapsed().as_secs_f64() < 2.0, "took {:?}", t.elapsed());
+        let text = |s: &mut Session, a: &str| s.execute("cell.get", json!({"cell": a})).unwrap()["text"].clone();
+        assert_eq!(text(&mut s, "B1"), "A2");
+        let last = CellRef::new(0, 3003).a1();
+        assert_eq!(text(&mut s, &last), "x3000");
+    }
+
     #[test]
     fn hostile_params_never_panic() {
         let mut s = sales();
diff --git a/crates/engine/src/pivot.rs b/crates/engine/src/pivot.rs
index 02a77f246..fa85ae959 100644
--- a/crates/engine/src/pivot.rs
+++ b/crates/engine/src/pivot.rs
@@ -126,18 +126,25 @@ pub fn read_source(wb: &Workbook, text: &str, default_sheet: usize) -> Result<So
         return Err("The source range is too large for a PivotTable.".into());
     }
     let mut headers: Vec<String> = Vec::new();
+    // Names taken so far (lower-case), and the next suffix to try for each repeated header, so
+    // that thousands of equal headers don't take cubic time.
+    let mut taken: HashSet<String> = HashSet::new();
+    let mut next_suffix: HashMap<String, usize> = HashMap::new();
     let mut formats = Vec::new();
     for c in r.start.col..=r.end.col {
         let h = crate::display::cell_text(wb, sh, CellRef::new(r.start.row, c)).trim().to_string();
         if h.is_empty() {
             return Err(FIELD_NAME_ERR.into());
         }
+        let key = h.to_ascii_lowercase();
         let mut n = h.clone();
-        let mut k = 2;
-        while headers.iter().any(|x| x.eq_ignore_ascii_case(&n)) {
+        let mut k = next_suffix.get(&key).copied().unwrap_or(2);
+        while taken.contains(&n.to_ascii_lowercase()) {
             n = format!("{h}{k}");
             k += 1;
         }
+        next_suffix.insert(key, k);
+        taken.insert(n.to_ascii_lowercase());
         headers.push(n);
         let fmt = if r.end.row > r.start.row {
             wb.styles.get(sh.style_id(CellRef::new(r.start.row + 1, c))).num_fmt.as_str().to_string()
diff --git a/crates/xlsx/src/pivot.rs b/crates/xlsx/src/pivot.rs
index 656af939b..3e6e24154 100644
--- a/crates/xlsx/src/pivot.rs
+++ b/crates/xlsx/src/pivot.rs
@@ -415,17 +415,22 @@ fn build_cache(wb: &Workbook, pt: &PivotTable, pivot_sheet: usize) -> Option<Cac
     }
     let sh = wb.sheet(si)?;
     let mut fields: Vec<CField> = Vec::new();
+    // Lower-case names taken, and the next suffix for each header (linear for repeated headers).
+    let mut taken = std::collections::HashSet::new();
+    let mut next_suffix: HashMap<String, usize> = HashMap::new();
     for (k, c) in (r.start.col..=r.end.col).enumerate() {
         let mut h = header_text(&sh.value(CellRef::new(r.start.row, c)).scalar());
         if h.is_empty() {
             h = format!("Column{}", k + 1);
         }
         let mut n = h.clone();
-        let mut i = 2;
-        while fields.iter().any(|f| same(&f.name, &n)) {
+        let mut i = next_suffix.get(&h.to_lowercase()).copied().unwrap_or(2);
+        while taken.contains(&n.to_lowercase()) {
             n = format!("{h}{i}");
             i += 1;
         }
+        next_suffix.insert(h.to_lowercase(), i);
+        taken.insert(n.to_lowercase());
         fields.push(CField {
             name: n,
             col: Some(k as u32),