Performance regression with v1

[zkat]

Heya. I noticed a significant (~2x) perf regression after upgrading to v1 recently. I also tested against 1.1.0 and didn't see a difference. The regression happens when doing filesystem reads (though it may happen in other situations -- I'm only sharing what my own benchmarks do).

Repro

1. git clone https://github.com/zkat/cacache-rs --branch async-std-regression
2. cd cacache-rs
3. cargo bench
4. git checkout head~1
5. cargo bench

Details

You can see the benchmarks themselves over at https://github.com/zkat/cacache-rs/blob/async-std-regression/benches/benchmarks.rs#L44-L74