See Advanced Render Settings in the Arnold User Guide

Lock Sampling Pattern

Locks the AA_seed so that the sampling noise won't change with the frame number (avoiding the film grain look).

Use Autobump in SSS (deprecated)

If enabled, autobump is visible in SSS. Enabling this option will take into account the effect that displacement_autobump has on the ray-traced BSSRDF's result. This helps capture the high-frequency details of the surface more accurately when using autobump.

Beware that enabling this option will triple shader evaluations with SSS, resulting in much longer render times.

Nested Dielectrics

Enables the nested dielectrics system for IOR tracking in scenes with adjacent dielectrics. Defaults to on.

Stochastic Volume Interpolation

The tricubic interpolation mode of AiVolumeSampleXXX() calls, such as those used by the standard_volume shader, now uses a stochastic method of interpolation that greatly reduces the amount of voxel data read in each sample call. This can speed up rendering by as much as 20% in assets such as the WDAS cloud data set. Note that this technique does not always produce a perfectly matching image as a result, and can be toggled via the stochastic_volume_interpolation render option (default is on).

Procedural Optimization

Conservative and Exhaustive modes offer optimizations that enhance the rendering speed of procedurals. Exhaustive mode performs all possible types of optimizations to achieve faster rendering, even if it may result in higher memory usage. On the other hand, Conservative mode applies only those optimizations that boost performance without increasing memory usage. Additionally, the None mode can be used to disable these optimizations and revert to the behavior of previous versions of Arnold.

Indirect Specular Blur

Sets indirect specular blurring to reduce caustic noise. Setting to zero gives the most accurate but also noisy renders, while higher values blur caustics to reduce noise.