diff --git a/markdown_it/rules_inline/fragments_join.py b/markdown_it/rules_inline/fragments_join.py
index f795c136..5eb88a14 100644
--- a/markdown_it/rules_inline/fragments_join.py
+++ b/markdown_it/rules_inline/fragments_join.py
@@ -29,14 +29,25 @@ def fragments_join(state: StateInline) -> None:
             and curr + 1 < maximum
             and state.tokens[curr + 1].type == "text"
         ):
-            # collapse two adjacent text nodes
-            state.tokens[curr + 1].content = (
-                state.tokens[curr].content + state.tokens[curr + 1].content
-            )
-        else:
-            if curr != last:
-                state.tokens[last] = state.tokens[curr]
+            # Collapse a run of adjacent text nodes in a single join, instead
+            # of pairwise `a + b` concatenation. The pairwise form is O(L*k)
+            # in the size of the run because each step rebuilds the growing
+            # prefix; "".join is O(L).
+            parts = [state.tokens[curr].content]
+            curr += 1
+            while curr < maximum and state.tokens[curr].type == "text":
+                parts.append(state.tokens[curr].content)
+                curr += 1
+            merged = state.tokens[curr - 1]
+            merged.content = "".join(parts)
+            merged.level = level
+            state.tokens[last] = merged
             last += 1
+            continue
+
+        if curr != last:
+            state.tokens[last] = state.tokens[curr]
+        last += 1
         curr += 1
 
     if curr != last: