Running lsof on external volume takes several minutes

[yk4to]

On a USB-connected SSD (5Gbps max), it took more than 3 minutes to identify the application that was using a certain file.

The +D option of lsof takes a very long time to execute because it recursively checks all files in a directory.

macOS detects the processes by some mechanism which is probably a low layer such as a file descriptor, so it may be necessary to use C APIs to get close to that.

related: #8