There is one check box left in the reflection parameters that I
didn’t explain. The ‘use interpolation’ check box. This can be used
to speed up the calculation of glossy reflections. It works more
or less the same way as the Irradiance map for calculating GI.
When you check this box, you can adjust interpolation settings in one of the other V-Ray material rollouts.
I will not explain these settings, because I don’t recommend using the interpolation for glossy reflections. They rarely look good, and when you want quality, they take almost as long as the standard glossy reflections.