Assuming these systems are running on your home network which is probably a residential internet connection. One of the huge problems with a residential connection is they are asynchronous which means you have a very small upload pipe which will also affect inbound traffic to anything on your network such as the FTP server. The upload speed is best effort and even if a speed test shows one thing you likely won't even see that speed most times. Likely your only, or at least best, answer is to call your service provider and upgrade to a business class internet connection. They are a bit pricier but you get a coup of important perks. 1. being the synchronous connect speeds and 2. you are more or less guaranteed the speed you pay for...