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Turns out that io.Discard is implementing ReadFrom using a small buffer (8192), confusing our benchmarks. We copyBuffer with 1 MiB buffer, but io.Discard is using its own 8 KiB buffer to do huge amount of tiny reads. These tiny reads are extremely slow for reading compressed clusters, since we have to read and decompress the same cluster multiple times.
With this change qcow2 zlib performance is 4 times better - it it still slow, but matches better the real performance.
Before:
After: