You can set any of the following options to optimize the render of motion blur.
Reduce the scene's shutter Speed to speed up motion blur calculations.
Select mental ray's Scanline or Rasterizer algorithms to render a faster motion blur than is done with raytracing.
If your scene has a lot of motion blur, you may need to increase the sampling in mental ray's Rendering 
Aliasing options.
Try a value such as 0, 2 or 0, 3 (Min and Max Levels). However, before you increase these sampling settings, first try adjusting the Motion Samples.
Use mental ray's Motion Samples to fine-tune the motion blur effect instead of the Sampling Contrast. You can achieve good quality with little impact on render performance.
Use the mental ray production 2D Motion Blur and Motion Vector (mip) shaders, or any third-party 2D motion blur plug-in instead of raytraced or scanline motion blur algorithms. 2D motion blur is handled as an output shader and applies the motion blur effect after the image is rendered.
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