It will always be many times lower than your maximum available download speed.
That 100KB/s is probably a false representation. When your computer connects to the FTP site and data starts transferring, there may be a delay before the calculated transfer rate is displayed. In the meantime, data has gone, and so the process calculates the total data transferred compared to the time when the display appears.
When everything is up and running, the display speed settles down to the true transfer rate.
I have tried to use the Ftp but it still very slow, first the upload speed of my internet reach 100KB/s, then it slow down to 18KB/s then 7KB/s, why is this happen? How to find out what is our ISP speed limit for Upload speed through our computer internet controller panel?Please help. Thanks