System Information
Windows 10 64 bit
I7, nvidia 1060

Blender Version
2.79 RC1

Short description of error
A simple character with an armature modifier, followed by a subidivision modifier with OpenSubDiv selected after it causes bone selection in pose mode to slow to a grind. If opensubdiv is unchecked, bones can be selected very quickly. If the subdivision modifier is moved above the armature, bone selection is quick and responsive, although the deformation is slower.

Exact steps for others to reproduce the error
Select armature, pose mode. Select different bones. Now turn off "use open subdivision". Bones can be selected quickly and responsively.