The compression-level can be set in the range of -1..9, with 6 being the default compression level. Levels above 6 will have higher compression rates at the cost of more CPU and memory usage. Levels below 6 will use less CPU and memory at the cost of lower compression rates.
final int level
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https://api.dartlang.org/stable/2.0.0/dart-io/ZLibEncoder/level.html