I am looking for an FTP client that works on Windows 7 and can open a folder with millions of files quickly (e.g. 50 millions). It should be GUI and list all the files. Both FileZilla and WinSCP takes forever to display the folder's content.
I already replied to your similar question here:
Specifically in this question (FTP client), also network data transfer speed comes in as significant factor – see comments of Thomas Weller under your question. But you can still attempt to put files on remote side into single archive (TAR, ZIP) and after transferring the archive to your local machine, you can unpack it and go with one of solutions sketched in the answer linked above.