Rendertimes with GI cache

[Hannes]

I have a scene. Interior. Kind of a space station (again... ). No sun, no atmosphere. Lighting only by self illuminated textures. I tried it with the path tracer, but it was noisy as hell, and the rendertimes were incredibly high with higher settings and still noisy. So I used the legacy renderer. It looks OK, but I badly miss the PT shader possibilities, especially for reflections.

However, here is my question: why does the exact same scene with a premade GI cache take 4-5 minutes per frame to render, and without premade GI cache less than one and a half minute? As I said, it's the same scene, no changes. I always thought a premade GI cache would reduce rendertimes, since the GI calculation is already made.

[WAS]

I am assuming this has to do with the caching of environment light rather than direct light. Does cache calculate shadows? Maybe the shadowing with bounce is causing the slowdown.

[Matt]

That's a surprising result. Could it be loading more than one GI cache file? Do you have the read settings set to "One file"? What is the render time for the pre-pass, and is it similar to the time it takes to pre-generate the cache file?

[Hannes]

It's an animation. So I created a sequence of GI cache files. And you're right. If I set the read settings to "one file", it takes just one and a half minute to render. If I don't use the cache file, the prepass takes less than 10 seconds for one frame. Generating the cache file is similar to that.
So the long rendertimes are because of reading more than one GI cache file and blending them. I didn't expect, that blending the premade cache files takes so long.