Win 8.1 x64, 3x gtx580, 2.70a

When rendering multiple layers in a batch animation with motion blur, some layers get extremely slow (compared to rendering one by one) and freeze Blender or whole system.

Open attached file, press render animation.
First (Mesh) layer renders fine, the second (floor/shadow) behaves like:

• On CPU - extremely slow (compared to rendering only this render layer. not to confuse with scene layer), cancellation takes couple of minutes.
• On all GPUs - system freeze (presumably it stays alive, but display gets no updates anymore).
• On single non-primary GPU - extremely slow, blocks don't get updated until complete. Render cancellation takes a minute or two.

This behavior is not observed if motion blur is turned off.
This behavior is not observed if render layers are rendered one by one, manually un-chec the others, even with motion blur on.

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