Performance improvement in :class:DatetimeArray and :class:DatetimeIndex: string formatting is now up to 80% faster (as fast as default) when one of the default strftime formats "%Y-%m-%d %H:%M:%S" or "%Y-%m-%d %H:%M:%S.%f" is used. See pandas-dev#44764

Author: Sylvain MARIE

doc/source/whatsnew/v1.5.0.rst

@@ -385,7 +385,8 @@ Performance improvements
 - Performance improvement when setting values in a pyarrow backed string array (:issue:`46400`)
 - Performance improvement in :func:`factorize` (:issue:`46109`)
 - Performance improvement in :class:`DataFrame` and :class:`Series` constructors for extension dtype scalars (:issue:`45854`)
-- Performance improvement in :class:`BusinessHour`, ``repr`` is now 4 times faster ! (related to :issue:`44764`)
+- Performance improvement in :class:`BusinessHour`, ``str`` and ``repr`` are now 4 times faster ! (related to :issue:`44764`)
+- Performance improvement in :class:`DatetimeArray` and :class:`DatetimeIndex`: string formatting is now up to 80% faster (as fast as default) when one of the default strftime formats ``"%Y-%m-%d %H:%M:%S"`` or ``"%Y-%m-%d %H:%M:%S.%f"`` is used. (:issue:`44764`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.bug_fixes:

pandas/_libs/tslib.pyx

@@ -145,6 +145,16 @@ def format_array_from_datetime(
                 consider_values //= 1000
                 show_ms = (consider_values % 1000).any()
 
+    elif format == "%Y-%m-%d %H:%M:%S":
+        # Same format as default, but with hardcoded precision (s)
+        basic_format = True
+        show_ns = show_us = show_ms = False
+
+    elif format == "%Y-%m-%d %H:%M:%S.%f":
+        # Same format as default, but with hardcoded precision (us)
+        basic_format = show_us = True
+        show_ns = show_ms = False
+
     for i in range(N):
         val = values[i]