diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 252d8966b7450f..824f58086a4fff 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -398,11 +398,12 @@ zlib
 Optimizations
 =============
 
-module_name
------------
-
-* TODO
+csv
+---
 
+* The :meth:`csv.Sniffer.sniff` delimiter detection has been optimized,
+  and is now up to 1.5x faster.
+  (Contributed by Maurycy Pawłowski-Wieroński in :gh:`137628`.)
 
 
 Deprecated
diff --git a/Lib/csv.py b/Lib/csv.py
index 0a627ba7a512fa..32ac6131d7dd0d 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -364,31 +364,36 @@ def _guess_delimiter(self, data, delimiters):
         try and evaluate the smallest portion of the data possible, evaluating
         additional chunks as necessary.
         """
+        from collections import Counter, defaultdict
 
         data = list(filter(None, data.split('\n')))
 
-        ascii = [chr(c) for c in range(127)] # 7-bit ASCII
-
         # build frequency tables
         chunkLength = min(10, len(data))
         iteration = 0
-        charFrequency = {}
+        num_lines = 0
+        # {char -> {count_per_line -> num_lines_with_that_count}}
+        charFrequency = defaultdict(Counter)
         modes = {}
         delims = {}
         start, end = 0, chunkLength
         while start < len(data):
             iteration += 1
             for line in data[start:end]:
-                for char in ascii:
-                    metaFrequency = charFrequency.get(char, {})
-                    # must count even if frequency is 0
-                    freq = line.count(char)
-                    # value is the mode
-                    metaFrequency[freq] = metaFrequency.get(freq, 0) + 1
-                    charFrequency[char] = metaFrequency
-
-            for char in charFrequency.keys():
-                items = list(charFrequency[char].items())
+                num_lines += 1
+                for char, count in Counter(line).items():
+                    if char.isascii():
+                        charFrequency[char][count] += 1
+
+            for char, counts in charFrequency.items():
+                items = list(counts.items())
+                missed_lines = num_lines - sum(counts.values())
+                if missed_lines:
+                    # charFrequency[char][0] can only be deduced now
+                    # as it cannot be obtained when parsing the lines.
+                    assert 0 not in counts.keys()
+                    # Store the number of lines 'char' was missing from.
+                    items.append((0, missed_lines))
                 if len(items) == 1 and items[0][0] == 0:
                     continue
                 # get the mode of the frequencies
diff --git a/Misc/NEWS.d/next/Library/2025-08-11-04-52-18.gh-issue-137627.Ku5Yi2.rst b/Misc/NEWS.d/next/Library/2025-08-11-04-52-18.gh-issue-137627.Ku5Yi2.rst
new file mode 100644
index 00000000000000..82111c1c2f2b28
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-08-11-04-52-18.gh-issue-137627.Ku5Yi2.rst
@@ -0,0 +1 @@
+Speed up :meth:`csv.Sniffer.sniff` delimiter detection by up to 1.5x.