diff --git a/pandas/_libs/reduction.pyx b/pandas/_libs/reduction.pyx
index 3a0fda5aed620..9459cd297c758 100644
--- a/pandas/_libs/reduction.pyx
+++ b/pandas/_libs/reduction.pyx
@@ -1,7 +1,5 @@
 from copy import copy
 
-from cython import Py_ssize_t
-
 from libc.stdlib cimport free, malloc
 
 import numpy as np
@@ -11,14 +9,14 @@ from numpy cimport int64_t, ndarray
 
 cnp.import_array()
 
-from pandas._libs cimport util
+from pandas._libs.util cimport is_array, set_array_not_contiguous
 
 from pandas._libs.lib import is_scalar, maybe_convert_objects
 
 
 cpdef check_result_array(object obj, Py_ssize_t cnt):
 
-    if (util.is_array(obj) or
+    if (is_array(obj) or
             (isinstance(obj, list) and len(obj) == cnt) or
             getattr(obj, 'shape', None) == (cnt,)):
         raise ValueError('Must produce aggregated value')
@@ -33,7 +31,7 @@ cdef class _BaseGrouper:
         if (dummy.dtype != self.arr.dtype
                 and values.dtype != self.arr.dtype):
             raise ValueError('Dummy array must be same dtype')
-        if util.is_array(values) and not values.flags.contiguous:
+        if is_array(values) and not values.flags.contiguous:
             # e.g. Categorical has no `flags` attribute
             values = values.copy()
         index = dummy.index.values
@@ -106,7 +104,7 @@ cdef class SeriesBinGrouper(_BaseGrouper):
         self.f = f
 
         values = series.values
-        if util.is_array(values) and not values.flags.c_contiguous:
+        if is_array(values) and not values.flags.c_contiguous:
             # e.g. Categorical has no `flags` attribute
             values = values.copy('C')
         self.arr = values
@@ -204,7 +202,7 @@ cdef class SeriesGrouper(_BaseGrouper):
         self.f = f
 
         values = series.values
-        if util.is_array(values) and not values.flags.c_contiguous:
+        if is_array(values) and not values.flags.c_contiguous:
             # e.g. Categorical has no `flags` attribute
             values = values.copy('C')
         self.arr = values
@@ -288,9 +286,9 @@ cpdef inline extract_result(object res, bint squeeze=True):
         res = res._values
         if squeeze and res.ndim == 1 and len(res) == 1:
             res = res[0]
-    if hasattr(res, 'values') and util.is_array(res.values):
+    if hasattr(res, 'values') and is_array(res.values):
         res = res.values
-    if util.is_array(res):
+    if is_array(res):
         if res.ndim == 0:
             res = res.item()
         elif squeeze and res.ndim == 1 and len(res) == 1:
@@ -304,7 +302,7 @@ cdef class Slider:
     """
     cdef:
         ndarray values, buf
-        Py_ssize_t stride, orig_len, orig_stride
+        Py_ssize_t stride
         char *orig_data
 
     def __init__(self, ndarray values, ndarray buf):
@@ -316,11 +314,9 @@ cdef class Slider:
 
         self.values = values
         self.buf = buf
-        self.stride = values.strides[0]
 
+        self.stride = values.strides[0]
         self.orig_data = self.buf.data
-        self.orig_len = self.buf.shape[0]
-        self.orig_stride = self.buf.strides[0]
 
         self.buf.data = self.values.data
         self.buf.strides[0] = self.stride
@@ -333,10 +329,8 @@ cdef class Slider:
         self.buf.shape[0] = end - start
 
     cdef reset(self):
-
-        self.buf.shape[0] = self.orig_len
         self.buf.data = self.orig_data
-        self.buf.strides[0] = self.orig_stride
+        self.buf.shape[0] = 0
 
 
 class InvalidApply(Exception):
@@ -408,39 +402,34 @@ cdef class BlockSlider:
     """
     Only capable of sliding on axis=0
     """
-
-    cdef public:
-        object frame, dummy, index
-        int nblocks
-        Slider idx_slider
-        list blocks
-
     cdef:
+        object frame, dummy, index, block
+        list blk_values
+        ndarray values
+        Slider idx_slider
         char **base_ptrs
+        int nblocks
+        Py_ssize_t i
 
     def __init__(self, object frame):
-        cdef:
-            Py_ssize_t i
-            object b
-
         self.frame = frame
         self.dummy = frame[:0]
         self.index = self.dummy.index
 
-        self.blocks = [b.values for b in self.dummy._mgr.blocks]
+        self.blk_values = [block.values for block in self.dummy._mgr.blocks]
 
-        for x in self.blocks:
-            util.set_array_not_contiguous(x)
+        for values in self.blk_values:
+            set_array_not_contiguous(values)
 
-        self.nblocks = len(self.blocks)
+        self.nblocks = len(self.blk_values)
         # See the comment in indexes/base.py about _index_data.
         # We need this for EA-backed indexes that have a reference to a 1-d
         # ndarray like datetime / timedelta / period.
         self.idx_slider = Slider(
             self.frame.index._index_data, self.dummy.index._index_data)
 
-        self.base_ptrs = <char**>malloc(sizeof(char*) * len(self.blocks))
-        for i, block in enumerate(self.blocks):
+        self.base_ptrs = <char**>malloc(sizeof(char*) * self.nblocks)
+        for i, block in enumerate(self.blk_values):
             self.base_ptrs[i] = (<ndarray>block).data
 
     def __dealloc__(self):
@@ -450,10 +439,9 @@ cdef class BlockSlider:
         cdef:
             ndarray arr
             Py_ssize_t i
-
         # move blocks
         for i in range(self.nblocks):
-            arr = self.blocks[i]
+            arr = self.blk_values[i]
 
             # axis=1 is the frame's axis=0
             arr.data = self.base_ptrs[i] + arr.strides[1] * start
@@ -470,10 +458,8 @@ cdef class BlockSlider:
         cdef:
             ndarray arr
             Py_ssize_t i
-
-        # reset blocks
         for i in range(self.nblocks):
-            arr = self.blocks[i]
+            arr = self.blk_values[i]
 
             # axis=1 is the frame's axis=0
             arr.data = self.base_ptrs[i]