diff --git a/Lib/csv.py b/Lib/csv.py
index 70c53ae7791e34..4ad565f5bff38d 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -214,12 +214,18 @@ def _guess_quote_and_delimiter(self, data, delimiters):
         this way.
         """
 
+        # The body of a quoted field ends at the first quote which is
+        # not doubled, as it does for a reader.  A lazy ".*?" scans to
+        # the end of the sample instead, from every start: quadratically.
+        # 2.7's re has no possessive "*+", so the body is matched
+        # atomically with a lookahead and a backreference.
+        body = r'(?=(?P<body>(?:(?P=quote){2}|(?!(?P=quote)).)*))(?P=body)'
         matches = []
-        for restr in ('(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?P=delim)', # ,".*?",
-                      '(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?P<delim>[^\w\n"\'])(?P<space> ?)',   #  ".*?",
-                      '(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?:$|\n)',   # ,".*?"
-                      '(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?:$|\n)'):                            #  ".*?" (no delim, no space)
-            regexp = re.compile(restr, re.DOTALL | re.MULTILINE)
+        for restr in (r'(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\'])%s(?P=quote)(?P=delim)',   # ,"...",
+                      r'(?:^|\n)(?P<quote>["\'])%s(?P=quote)(?P<delim>[^\w\n"\'])(?P<space> ?)',     #  "...",
+                      r'(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\'])%s(?P=quote)(?:$|\n)',  # ,"..."
+                      r'(?:^|\n)(?P<quote>["\'])%s(?P=quote)(?:$|\n)'):                           #  "..." (no delim, no space)
+            regexp = re.compile(restr % body, re.DOTALL | re.MULTILINE)
             matches = regexp.findall(data)
             if matches:
                 break
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 88d5af7c130d5d..e4c18ffe0a3a11 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -1096,6 +1096,13 @@ def test_doublequote(self):
         dialect = sniffer.sniff(self.sample9)
         self.assertTrue(dialect.doublequote)
 
+    def test_sniff_quoted_single_column(self):
+        # gh-98820: this sample used to take minutes.
+        sniffer = csv.Sniffer()
+        sample = '"abcdefghijklmnopqrstuvwxyz"\n' * 10000
+        with self.assertRaisesRegexp(csv.Error, "Could not determine delimiter"):
+            sniffer.sniff(sample, delimiters=',:|\t')
+
 class NUL:
     def write(s, *args):
         pass