There are occasions where Windows' behavior of trying to connect using WebDAV first before SMB/CIFS causes a long delay. The easiest way to fix this is to just disable the WebClient service in Windows. You can do this from Windows by going to Control Panel → Administrative Tools → Services. From there, open the WebClient service, stop it, and also select the option to disable it at startup.
It is also possible that the ReadyNAS disks were spun-down for power savings. There would be an initial delay of 4-12 seconds when they spin up.