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When attempting to launch KiteDemo recently, this operation required hours to complete (on a decent spec PC). It looks as if there's possible scope for improving the FAllocator2D algorithm (maintaining a list of overlapping unused rectangular regions, instead of a brute force O(n^4) approach which tests each pixel in turn).
Launch KiteDemo with no built assets. Static mesh HillTree_02 will take an hour or more to build.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-16956 in the post.