Increase overall rendering speed in mental ray for Maya

Diagnose the scene to find ways to render the scene faster using Render > Run render diagnostics. You can use this tool to monitor how well you optimize the scene and to search for limitations and potential problems that may occur. For more information on render diagnostics, see Run diagnostics and mental ray for Maya diagnostics.
Perform scene optimizations:

Click File > Optimize Scene Size > to turn options on or off to optimize everything in the scene and to remove unused or non-valid elements. See File > Optimize Scene Size for more information about this window.
For Maya software rendering, use Block ordered texture set up. See Cache texture tiles using BOT (block ordered texture).
If the scene contains objects with construction history and you no longer need it, delete it. See the Construction history in the Basics guide for details.

Avoid memory swapping by:

Closing all applications before rendering to maximize the amount of memory available for rendering (including Maya if rendering from a shell or command line).
Setting the TEMP or TMPDIR variable as the location for temporary render cache files: -TMPDIR (Linux) or - TEMP (Windows and Mac OS X) to make plenty of room for temporary rendered files. Make sure that the value of those variables points to a local, fast hard drive, not a network drive.

For Maya software and mental ray for Maya, Test Resolution (Render > Test Resolution) lets you select a reduced resolution to test render the scene. For more information on test rendering strategies, see Visualize interactively with IPR.
For Maya software, if the scene contains several identical surfaces (for example, multiple spheres), use Optimize Instances in the Render Settings: Maya Software tab to improve rendering performance.
Turn off motion blur if you don’t need it (the Vector renderer has no motion blur). For the Maya software renderer, use 2D motion blur instead of 3D motion blur when possible. See 2D Motion Blur global attributes and 3D Motion Blur in the Render Settings window for details.

Use average BSP (mental ray for Maya) settings

When mental ray for Maya raytraces, it calculates the effects using an average of the depth and the average of the leaf size settings of the BSP.

To speed up subsequent renderings, you can render instead with the average settings (instead of your initial settings).

Find the averages in Maya's Output Window after you render the scene the first time (with 'progress messages' turned on), then change the settings in the Memory and Performance section.

Increase overall rendering speed in mental ray for Maya

To make a scene render faster

Use average BSP (mental ray for Maya) settings

Example averages found in the Output Window