FTP filezilla and other programs do not work well how to determine the exact cause?

Question

FTP

Something recently, FTP is barely working, and Filezilla and Total Commander and other programs have tried WinCP, it seems that files of several kb are loaded in 5 10 minutes sometimes (often) repeatedly 6 7 times and it is often written that the file could not be uploaded, what can be the reason how to determine? and if the reason is in the provider how to explain? how to prove, they almost always answer that it’s not them until they point out the facts

Answer 1

Maybe there are restrictions on the download speed on the server, try testing one large file for downloading