diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 7fe2d936506..6954540b5da 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -26,7 +26,7 @@
 
 from . import (TestCase, requires_scipy, requires_netCDF4, requires_pydap,
                requires_scipy_or_netCDF4, requires_dask, requires_h5netcdf,
-               requires_pynio, has_netCDF4, has_scipy, assert_allclose)
+               requires_pynio, has_netCDF4, has_scipy, assert_allclose, slow)
 from .test_dataset import create_test_data
 
 try:
@@ -1112,16 +1112,19 @@ def test_3_autoclose_pynio(self):
 
     @requires_dask
     @requires_netCDF4
+    @slow
     def test_1_open_large_num_files_netcdf4(self):
         self.validate_open_mfdataset_large_num_files(engine=['netcdf4'])
 
     @requires_dask
     @requires_scipy
+    @slow
     def test_2_open_large_num_files_scipy(self):
         self.validate_open_mfdataset_large_num_files(engine=['scipy'])
 
     @requires_dask
     @requires_pynio
+    @slow
     def test_3_open_large_num_files_pynio(self):
         self.validate_open_mfdataset_large_num_files(engine=['pynio'])