As an aside, if it is safe to assume that the faces are not going to change
between digits of the line number, perhaps this change would be a gain.

It moves merging the tick faces outside the lnum_buf loop, and it is done
only for the right line numbers and when not beyond_zv.

diff --git i/src/xdisp.c w/src/xdisp.c
index ad73981c1d..2d79a42270 100644
--- i/src/xdisp.c
+++ w/src/xdisp.c
@@ -22657,6 +22657,21 @@ maybe_produce_line_number (struct it *it)
   int width_limit =
     tem_it.last_visible_x - tem_it.first_visible_x
     - 3 * FRAME_COLUMN_WIDTH (it->f);
+
+  /* Select face for tick line numbers, if needed */
+  int tick_face_id = -1;
+  if (!beyond_zv)
+    {
+      if (display_line_numbers_major_tick > 0
+     && (lnum_to_display % display_line_numbers_major_tick == 0))
+   tick_face_id = merge_faces (it->w, Qline_number_major_tick,
+                   0, DEFAULT_FACE_ID);
+      else if (display_line_numbers_minor_tick > 0
+          && (lnum_to_display % display_line_numbers_minor_tick == 0))
+   tick_face_id = merge_faces (it->w, Qline_number_minor_tick,
+                   0, DEFAULT_FACE_ID);
+    }
+
   /* Produce glyphs for the line number in a scratch glyph_row.  */
   for (const char *p = lnum_buf; *p; p++)
     {
@@ -22671,14 +22686,8 @@ maybe_produce_line_number (struct it *it)
           ? this_line == 0
           : this_line == it->pt_lnum))
    tem_it.face_id = current_lnum_face_id;
-      else if (display_line_numbers_major_tick > 0
-          && (lnum_to_display % display_line_numbers_major_tick == 0))
-   tem_it.face_id = merge_faces (it->w, Qline_number_major_tick,
-                     0, DEFAULT_FACE_ID);
-      else if (display_line_numbers_minor_tick > 0
-          && (lnum_to_display % display_line_numbers_minor_tick == 0))
-   tem_it.face_id = merge_faces (it->w, Qline_number_minor_tick,
-                     0, DEFAULT_FACE_ID);
+      else if (tick_face_id >= 0)
+   tem_it.face_id = tick_face_id;
       else
    tem_it.face_id = lnum_face_id;
       if (beyond_zv