PERF: Speeds up creation of Period, PeriodArray, with Offset freq (pandas-dev#23589)

Author: TomAugspurger

pandas/_libs/tslibs/offsets.pyx

@@ -84,6 +84,8 @@ cdef to_offset(object obj):
     Wrap pandas.tseries.frequencies.to_offset to keep centralize runtime
     imports
     """
+    if isinstance(obj, _BaseOffset):
+        return obj
     from pandas.tseries.frequencies import to_offset
     return to_offset(obj)
 

pandas/_libs/tslibs/period.pyx

@@ -1567,7 +1567,6 @@ cdef class _Period(object):
 
     @classmethod
     def _maybe_convert_freq(cls, object freq):
-
         if isinstance(freq, (int, tuple)):
             code, stride = get_freq_code(freq)
             freq = get_freq_str(code, stride)