Should I use meshes or proxies with Forest in V-Ray ?

Internally all Forest objects are converted to native instances of V-Ray, so there is no performance loss using meshes instead proxies.

Here is a brief list about pros and cons between meshes / proxies:

Meshes​

• The geometry is loaded in memory while all rendering time (only a copy by tree model). This takes more space in the max scene, although you can fix it using XRefs.
• Any geometric object can be used as source, including parametric and animated objects (Onyx, GrowFX or other Forest object).
• The animation samples are generated on the fly, allowing to create endless animations, but this may be slow depending on the type of object.
• Some Forest features as Tint by Element only can be used with objects

Proxies​

• The geometry is loaded/unloaded from disk dynamically, only when it is required by the current rendering bucket. This optimizes the use of the memory, but usually is slower.
• The proxies don't take space on the scene, but they must be created manually before rendering. Also, you must take care to make them visible to all render nodes.
• You can store the animation on the proxy file. It is much faster to load the samples from disk, than generate them at each frame.