(Windows) Similar files with vastly different transfer speeds

[SyUser044]

I am a Resilio Sync user for some time now, which I use to remotely transfer files from my home country to the one I currently reside without the hassle of a FTP server. Recently, I stumbled upon a rather bewildering issue of which I am unable to make sense. Imagine one has four folders, each with 6-10 .mkv files in the size range of 800-1200 MB. Since all the files are of the same type and with similar sizes, the transfer of each of those folders should have more or less the same speed, all else equal. However, that is not what I am experiencing. One of the folders, the one first added, had a transfer rate that far exceeds all others: 2 MB/s against 30 KB/s. That is mighty strange considering that if I pause it on both ends and allow only one of the other folders to synchronize, their speed remains exactly the same as to when they were simultaneously running. In fact, at first I thought my connection was at fault because I was running only one of those slow folders in the beginning. Only after exhausting all options and deciding what harm would come from letting all of them to transfer simultaneously that I discovered this oddity. 

Does anyone have any idea as to why those folders are being transferred at different rates?