Constant READ_PREFETCH_DEPTH 

const READ_PREFETCH_DEPTH: usize = 8;

Number of segments a read fetches ahead of the one whose entries it yields. On S3 a replay that fetched each segment after the previous one spent about 30 ms per object; with eight fetches in flight a segment arrives about every 4 ms. The segments fetched ahead also take their bytes from READ_PREFETCH_BYTES.