Ok, I think I know what is happening. First of all, PT+PT works as expected - higher albedo = slower rendering, no surprises here.
PT+UHD is different story. Maru is right - low albedo image has higher contrast, so it takes longer. But lets elaborate. Each rendered image is made by adding up DIRECT and INDIRECT components. In PT+UHD, the DIRECT pass is calculated with path tracing, and the INDIRECT pass is calculated with UHD cache. UHD cache is MUCH faster than path tracing and produces very low noise. But it cannot be used for direct pass because it could not handle its contrast.
First lets compare the result images:
Low albedo image is obviously much darker, but also computes longer. Why? Lets decompose the images to DIRECT and INDIRECT parts:
Direct:
Indirect:
Notice anything?
1) Direct passes have roughly the same brightness
2) Direct pass in high albedo image is MUCH noisier
3) Indirect pass with high albedo is MUCH brighter
4) Indirect passes have roughly the same noise
Explanation:
1) + 3) direct lighting is conceptually computed as emission*albedo, but indirect lighting is emission*albedo*albedo + emission*albedo*albedo*albedo + emission*albedo*albedo*albedo + .... - so the albedo plays much bigger role in indirect illumination calculation. This makes the result much brighter with high albedo, and also makes it compute longer when not using caching
2) This is because UHD cache does not help with direct light computation, and the high albedo image has fewer passes
4) This is because thanks to UHD cache it basically does not matter what the albedo or scene complexity is - it will always compute fast
Now why the high albedo image stopped calculating earlier? Because as I said earlier, the resulting image is DIRECT + INDIRECT. Indirect is always noise-free and cheap to compute thanks to UHD cache. Direct is noisy. So you add noisy DIRECT + clean INDIRECT and compute the whole image noise from it. In the high-albedo case, the INDIRECT pass is much brighter and influences the result more. Basically it masks the direct pass noise out from the image. In low-albedo case, the indirect pass is weak and the noise from direct pass can be seen over it clearly, so it needs to go away so the whole image looks noise-free.
Low albedo:
noisy direct + weak clean indirect = noisy result
High albedo:
noisy direct + strong clean indirect = clean result
