diff --git a/src/uu/sort/src/merge.rs b/src/uu/sort/src/merge.rs
index 85276fce6d8..8900284e82f 100644
--- a/src/uu/sort/src/merge.rs
+++ b/src/uu/sort/src/merge.rs
@@ -30,7 +30,7 @@ use uucore::error::{FromIo, UResult};
 use crate::{
     GlobalSettings, Output, SortError,
     chunks::{self, Chunk, RecycledChunk},
-    compare_by, current_open_fd_count, fd_soft_limit, open,
+    current_open_fd_count, fd_soft_limit, merge_compare, open,
     tmp_dir::TmpDirWrapper,
 };
 
@@ -200,7 +200,13 @@ fn merge_without_limit<M: MergeInput + 'static, F: Iterator<Item = UResult<M>>>(
     }
 
     let reader_join_handle = thread::spawn({
-        let settings = settings.clone();
+        // The merge comparator (`merge_compare`) compares whole-line locale keys lazily
+        // with the ICU collator, so the reader does not need to precompute per-line sort
+        // keys. Disabling `fast_locale_collation` here turns `Line::create` into a no-op
+        // for that (whole-line, default) mode and avoids the dominant cost of merging
+        // already-sorted input. Other modes are unaffected (the flag is already false).
+        let mut settings = settings.clone();
+        settings.precomputed.fast_locale_collation = false;
         move || {
             reader(
                 &request_receiver,
@@ -332,7 +338,7 @@ impl FileMerger<'_> {
                 let current_line = &contents.lines[file.line_idx];
                 if settings.unique {
                     if let Some(prev) = &prev {
-                        let cmp = compare_by(
+                        let cmp = merge_compare(
                             &prev.chunk.lines()[prev.line_idx],
                             current_line,
                             settings,
@@ -383,7 +389,7 @@ struct FileComparator<'a> {
 
 impl Compare<MergeableFile> for FileComparator<'_> {
     fn compare(&self, a: &MergeableFile, b: &MergeableFile) -> Ordering {
-        let mut cmp = compare_by(
+        let mut cmp = merge_compare(
             &a.current_chunk.lines()[a.line_idx],
             &b.current_chunk.lines()[b.line_idx],
             self.settings,
diff --git a/src/uu/sort/src/sort.rs b/src/uu/sort/src/sort.rs
index e745365591a..53690e0264d 100644
--- a/src/uu/sort/src/sort.rs
+++ b/src/uu/sort/src/sort.rs
@@ -2621,6 +2621,37 @@ fn sort_by<'a>(unsorted: &mut Vec<Line<'a>>, settings: &GlobalSettings, line_dat
     }
 }
 
+/// Comparison used by the merge path.
+///
+/// This is result-identical to [`compare_by`], but for the whole-line locale-collation
+/// case it compares the two lines lazily with the ICU collator instead of relying on
+/// precomputed collation keys. Merging only performs O(n log k) comparisons (and none at
+/// all when merging a single file), so computing a full sort key for every line — as the
+/// regular sort path does to amortize O(n log n) comparisons — is pure overhead here.
+/// Skipping that per-line work (see `merge_without_limit`) is what makes `sort -m` of
+/// already-sorted input fast.
+pub fn merge_compare<'a>(
+    a: &Line<'a>,
+    b: &Line<'a>,
+    settings: &GlobalSettings,
+    a_line_data: &LineData<'a>,
+    b_line_data: &LineData<'a>,
+) -> Ordering {
+    #[cfg(feature = "i18n-collator")]
+    if settings.precomputed.fast_locale_collation {
+        // Mirror the `fast_locale_collation` branch of `compare_by`, but compare the line
+        // bytes directly rather than precomputed keys: `locale_cmp` (ICU `compare_utf8`)
+        // and the sort-key comparison agree on ordering by construction.
+        let mut cmp = locale_cmp(a.line, b.line);
+        if cmp == Ordering::Equal {
+            // Equal keys for inputs like `01` and `0_1`; fall back to (reversed) byte order.
+            cmp = b.line.cmp(a.line);
+        }
+        return if settings.reverse { cmp.reverse() } else { cmp };
+    }
+    compare_by(a, b, settings, a_line_data, b_line_data)
+}
+
 fn compare_by<'a>(
     a: &Line<'a>,
     b: &Line<'a>,