I have found the solution to this problem. It seems that Server 2019 uses a technique called "receive segment coalescing (RSC)". Server 2008R2 and Windows 7 seem to have trouble with transfer speeds if this function is enabled on Server 2019. I used the following command on the 2019 server using Windows Powershell:
You can check if this function is enabled on your Server 2019 server:
Get-NetAdapterRsc * | FL
after that verify it has been enabled you can run the following command:
Disable-NetAdapterRsc *
This disables the RSC function on the LAN adapter of the 2019 server. After this the 2008R2 server now has a much faster connection sending and receiving data to and from 2019 server. Maybe this isn't the best solution but it works great. The transfer speed in my case went from 8 MB/s to 109 MB/s