The main factor seems to be the best-fit allocator, which tends to go into action sooner from metaslabs crossing the 94% threshold earlier than the pool itself

The main factor seems to be the best-fit allocator, which tends to go into action sooner from metaslabs crossing the 94% threshold earlier than the pool itself and still be selected due to LBA weighting, which is a trick to increase throughput on rotational media. This ought to help prevent best-fit allocation from occurring earlier than necessary on SSDs: https://github.com/zfsonlinux/zfs/commit/fb40095f5f0853946f8150481ca22602d1334dfe https://github.com/zfsonlinux/zfs/commit/fb40095f5f0853946f8... That said, Delphix made changes to consider on fragmentation rather than space usage when the histogram is in use, so the performance before best-fit behavior goes into effect is better than outright selection of metaslabs by free space.