Hi KyL,
Do you find this to be the case when the surface layers generate only colour? Are you using fractals to generate colour and/or displacement?
Unfortunately the path tracer needs to evaluate the surface colour and any other BRDF attributes generated by the shader at each bounce. If these are tied to complex functions such as fractals or Voronoi functions, this can end up being a large contributor to the render time. Displacements also need to be computed multiple times for each ray. The Subdiv Cache helps when rays hit quite close to the point they originate from, but for long rays the cache becomes ineffective and displacement shaders can end up being a large contributor to render time.
The Standard Renderer can also be affected by this when you ray trace reflections and shadows. But in the path tracer these problems affect GI as well, so they have a larger impact.
I will create an option to use the GI Cache at high ray depths (optionally restricted to long range bounces). For scenes with complex shaders it will probably be much faster to render this way. If it's restricted to long range bounces, not short, it should not have much negative effect on the image quality.