issues: 485508509

MCVE Code Sample

```python import numpy as np import xarray as xr import pandas as pd import time

size = 1000000 data = np.random.random(size) times = pd.date_range('2019-01-01', periods=size, freq='ms') da = xr.DataArray(data, dims=['time'], coords={'time': times})

start = time.time()

da.resample(time='s').mean()

print('Elapsed time: ' + str(time.time() - start)) print('xarray version: ' + str(xr.version)) ```

• Output for xarray==0.11.3

Elapsed time: 0.2671010494232178 xarray version: 0.11.3

• Output for xarray==0.12.0

Elapsed time: 6.652455568313599 xarray version: 0.12.0

Expected Output

I expect that for the default parameters, resample should take a similar amount of time as previous versions. Or at least the documentation should specify what parameters must be passed into resample or mean to achieve the same time as in version 0.11.3

Problem Description

resampling a dataarray or dataset and then calling mean takes significantly longer in the latest versions of xarray 0.12. The changelog specifies multiple changes to the resample method and new parameters, but its not clear what one would specify to achieve execution time improvements.

Output of xr.show_versions()

# Paste the output here xr.show_versions() here xr.show_versions() gives me the following error `AttributeError: module 'h5py' has no attribute '__hdf5libversion__'` So I have just printed the version of xarray. I changed the xarray version by using `pip install xarray==0.11.3`. Here is my conda environment _anaconda_depends 2019.03 py37_0 _ipyw_jlab_nb_ext_conf 0.1.0 py37_0 alabaster 0.7.12 py37_0 anaconda custom py37_1 anaconda-client 1.7.2 py37_0 anaconda-navigator 1.9.7 py37_0 anaconda-project 0.8.3 py_0 asn1crypto 0.24.0 py37_0 astroid 2.2.5 py37_0 astropy 3.2.1 py37he774522_0 atomicwrites 1.3.0 py37_1 attrs 19.1.0 py37_1 babel 2.7.0 py_0 backcall 0.1.0 py37_0 backports 1.0 py_2 backports.functools_lru_cache 1.5 py_2 backports.os 0.1.1 py37_0 backports.shutil_get_terminal_size 1.0.0 py37_2 backports.tempfile 1.0 py_1 backports.weakref 1.0.post1 py_1 beautifulsoup4 4.7.1 py37_1 bitarray 0.9.3 py37he774522_0 bkcharts 0.2 py37_0 blas 1.0 mkl bleach 3.1.0 py37_0 blosc 1.16.3 h7bd577a_0 bokeh 1.2.0 py37_0 boto 2.49.0 py37_0 bottleneck 1.2.1 py37h452e1ab_1 bzip2 1.0.8 he774522_0 ca-certificates 2019.5.15 0 cantera 2.4.0 np115py37_2 cantera certifi 2019.6.16 py37_0 cffi 1.12.3 py37h7a1dbc1_0 chardet 3.0.4 py37_1 click 7.0 py37_0 cloudpickle 1.2.1 py_0 clyent 1.2.2 py37_1 colorama 0.4.1 py37_0 comtypes 1.1.7 py37_0 conda 4.7.5 py37_0 conda-build 3.18.7 py37_0 conda-env 2.6.0 1 conda-package-handling 1.3.11 py37_0 conda-verify 3.4.2 py_0 console_shortcut 0.1.1 3 contextlib2 0.5.5 py37_0 cryptography 2.7 py37h7a1dbc1_0 curl 7.64.1 h2a8f88b_0 cycler 0.10.0 py37_0 cython 0.29.12 py37ha925a31_0 cytoolz 0.10.0 py37he774522_0 dask 2.1.0 py_0 dask-core 2.1.0 py_0 decorator 4.4.0 py37_1 defusedxml 0.6.0 pypi_0 pypi distributed 2.1.0 py_0 docutils 0.14 py37_0 easygui 0.98.1 pypi_0 pypi entrypoints 0.3 py37_0 et_xmlfile 1.0.1 py37_0 fastcache 1.1.0 py37he774522_0 filelock 3.0.12 py_0 flask 1.1.1 py_0 freetype 2.9.1 ha9979f8_1 future 0.17.1 py37_0 get_terminal_size 1.0.0 h38e98db_0 gevent 1.4.0 py37he774522_0 gitdb2 2.0.5 pypi_0 pypi gitpython 2.1.11 pypi_0 pypi glob2 0.7 py_0 greenlet 0.4.15 py37hfa6e2cd_0 h5netcdf 0.7.4 pypi_0 pypi h5py 2.9.0 py37h5e291fa_0 hdf5 1.10.4 h7ebc959_0 heapdict 1.0.0 py37_2 html5lib 1.0.1 py37_0 icc_rt 2019.0.0 h0cc432a_1 icu 58.2 ha66f8fd_1 idna 2.8 py37_0 imageio 2.5.0 py37_0 imagesize 1.1.0 py37_0 importlib_metadata 0.17 py37_1 intel-openmp 2019.4 245 ipykernel 5.1.1 py37h39e3cac_0 ipython 7.6.1 py37h39e3cac_0 ipython_genutils 0.2.0 py37_0 ipywidgets 7.5.0 py_0 isort 4.3.21 py37_0 itsdangerous 1.1.0 py37_0 jdcal 1.4.1 py_0 jedi 0.13.3 py37_0 jinja2 2.10.1 py37_0 joblib 0.13.2 py37_0 jpeg 9b hb83a4c4_2 json5 0.8.4 py_0 jsonschema 3.0.1 py37_0 jupyter 1.0.0 py37_7 jupyter_client 5.3.1 py_0 jupyter_console 6.0.0 py37_0 jupyter_core 4.5.0 py_0 jupyterlab 1.0.2 py37hf63ae98_0 jupyterlab-git 0.6.1 pypi_0 pypi jupyterlab_server 1.0.0 py_0 keyring 18.0.0 py37_0 kiwisolver 1.1.0 py37ha925a31_0 krb5 1.16.1 hc04afaa_7 lazy-object-proxy 1.4.1 py37he774522_0 libarchive 3.3.3 h0643e63_5 libcurl 7.64.1 h2a8f88b_0 libiconv 1.15 h1df5818_7 liblief 0.9.0 ha925a31_2 libpng 1.6.37 h2a8f88b_0 libsodium 1.0.16 h9d3ae62_0 libssh2 1.8.2 h7a1dbc1_0 libtiff 4.0.10 hb898794_2 libxml2 2.9.9 h464c3ec_0 libxslt 1.1.33 h579f668_0 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