diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index dd29fbb1614aa8..740f18646df3f2 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -547,7 +547,7 @@ def commonpath(paths):
         split_paths = [path.split(sep) for path in paths]
 
         try:
-            isabs, = set(p[:1] == sep for p in paths)
+            isabs, = {p.startswith(sep) for p in paths}
         except ValueError:
             raise ValueError("Can't mix absolute and relative paths") from None
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-04-08-19-30-38.gh-issue-117641.oaBGSJ.rst b/Misc/NEWS.d/next/Core and Builtins/2024-04-08-19-30-38.gh-issue-117641.oaBGSJ.rst
new file mode 100644
index 00000000000000..e313c133b72173
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2024-04-08-19-30-38.gh-issue-117641.oaBGSJ.rst	
@@ -0,0 +1 @@
+Speedup :func:`os.path.commonpath` on Unix.