Number of concurrent SFTP requests that can be in-flight (sent but not yet acknowledged by the server) during file transfers.

Increasing this value improves transfer throughput significantly, especially on high-bandwidth or high-latency links, by keeping the network pipe full of data instead of waiting for each chunk to be acknowledged individually.

Default: 64. The library minimum is 1 (fully synchronous). Values above 128 may increase memory usage without meaningful throughput gains on most networks.