diff --git a/crates/forms/src/calc_order.rs b/crates/forms/src/calc_order.rs
index e75a05d47..428c1724d 100644
--- a/crates/forms/src/calc_order.rs
+++ b/crates/forms/src/calc_order.rs
@@ -122,6 +122,12 @@ pub(crate) fn run_order(doc: &Document, fields: &[Field]) -> (Vec<(usize, bool)>
     (run, extra)
 }
 
+#[cfg(test)]
+thread_local! {
+    /// Read names compared while merging a field's reads.
+    pub(crate) static NAMES_COMPARED: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
+}
+
 /// The form's calculation order (`fields` as [`fields`] reads them), with each calculated field's
 /// reads and issues, and Fix order's order.
 pub fn calculation_order(doc: &Document, fields: &[Field]) -> CalculationOrder {
@@ -132,8 +138,11 @@ pub fn calculation_order(doc: &Document, fields: &[Field]) -> CalculationOrder {
         .map(|(f, _)| {
             let mut names = f.actions.calculate.reads();
             if let Some(js) = &f.actions.scripts.calculate {
+                let mut known: HashSet<String> = names.iter().cloned().collect();
                 for name in crate::af::calculation_reads(js) {
-                    if !names.contains(&name) {
+                    #[cfg(test)]
+                    NAMES_COMPARED.with(|n| n.set(n.get() + 1));
+                    if known.insert(name.clone()) {
                         names.push(name);
                     }
                 }
diff --git a/crates/forms/src/tests.rs b/crates/forms/src/tests.rs
index b2e4994ec..2b2042865 100644
--- a/crates/forms/src/tests.rs
+++ b/crates/forms/src/tests.rs
@@ -2981,3 +2981,20 @@ fn page_walk_skips_repeated_and_cyclic_kids_without_queueing_them() {
     assert_eq!(walk.next(), None);
     assert_eq!(page_refs(&doc), [page]);
 }
+
+/// A Calculate script naming thousands of fields is merged with the field's other reads in one
+/// pass, not by searching the names so far for each one.
+#[test]
+fn a_script_reading_many_fields_merges_its_reads_in_one_pass() {
+    use crate::calc_order::NAMES_COMPARED;
+    let mut doc = one_page();
+    add_field(&mut doc, 0, [50.0, 700.0, 250.0, 720.0], &NewField::Text { multiline: false }, Some("total")).unwrap();
+    let n = 2_000;
+    let script: String = (0..n).map(|i| format!("getField(\"f{i}\").value + ")).collect::<String>() + "0";
+    set_field_script(&mut doc, "total", "calculate", Some(&format!("event.value = {script};"))).unwrap();
+    NAMES_COMPARED.with(|c| c.set(0));
+    let order = calculation_order(&doc, &fields(&doc));
+    assert_eq!(order.fields[0].reads.len(), n);
+    let compared = NAMES_COMPARED.with(std::cell::Cell::get);
+    assert!(compared <= 4 * n, "{compared} names compared for {n} reads");
+}