Slow FTP Upload Speed

Hello, my problem is: I have a new VPS in Contabo, and everytime I start an upload the transfer speed starts at 2MB/s and then slows down to 80KB/s, my upload link is 50Mb/s. This issue occurs when I"m uploading a big file, it"s like there is some limit, I have also tested to upload 10 big files at the same time, all the files have been uploaded at 80Kb/s each at the same time. I also have tested this in another computers and another ISP, all the same, but there is something even stranger. This 80KB/s upload rate is only in windows, when i upload the same file in the same ISP from linux, the upload rate starts at the same 2MB/s then slows down to 500KB/s, and if i upload 10 files, the same thing happens, all the 10 files uploads at 500KB/s each. The download speed is ok, 10MB/s, this issue is only in uploads, even when i upload a file by the cpanel file manager this upload speed is limited to 80KB/s. I have tested pureftp and proftp, same problem on booth, i have tested uploads via ftp, sftp and scp, same problem, i have tested many ftp clients like: filezilla, cuteftp, winscp and coreftp. I have also tested with speedtest-cli in ssh, the results was: 100Mb/s download, 100Mb/s Upload. My first guess was that it"s a firewall problem, but no, i have turned off the firewall, no changes. I also have a reseller acocunt and a shared host in Hostgator, in booth of then the upload speed is about 5MB/s to 8MB/s I don"t know what to do anymore. I really aprreciate if someone could help me to find a solutions for this. Best regards. Eduardo