Network switch file transfer

Hello,

I have set up two D-Link switches with trunking with two links between them, I have a windows machine plugged into each of them and I'm transferring 5GB between them to test the speed, for the first 15/20 seconds I have transfer speeds of about 125/140 MB/S  but then after that amount of time it drops off to about 49/50 is there any reason for this?

Thanks in advance!