How far away are the sites from each other. Are they on different ISP's.
Check a traceroute between site from public IP to Public IP.
Check the MTU between the sites.
what is the upload/download on each site.
Is it Fiber\DSL\COAX.
on the Slow side is the upload Lower then the fast side?
If the path is between multiple ISP's most likely the issue.
Also try with IPS\AMP disabled.
For testing, what kind of device is used for testing. Try different devices on each side, and only test on hard wired.