i noticed that for almost every file that has to be uploaded there's (at least) one CWD command even if the files being uploaded belong to the same directory.

so if you have 100 files in directory foo there will be 100 commands "CWD foo" and this slows down the transfer. i think that bulk uploading first all the file in the same directory would speed up the transfer a lot.

moreover if CWD "/foo/bar/x/y" fails, ftp upload tries to CWD all the directories from the first to the last in the path (ie. CWD foo, CWD bar, CWD x, CWD y). Probably it will be more efficent to try the inverse sequence: CWD "/foo/bar/x/", CWD "/foo/bar", CWD "/foo" because 99% of time it fails on the latest directory in the path. This makes a big difference when there are many nested dirs.