I have a MacOS Server (10.12.6) set up running a VPN server at one location (site A). I have a Mac Pro (10.12.6) and Macbook Pro (10.14.5 beta 3) running at a second location (site B). Both sites have symmetrical gigabit fiber internet (and yes, I am getting full speeds on each end). The two clients at site B have a VPN profile for the site A Server, which specifies all traffic be sent over the VPN. My Macbook Pro (wi-fi) is getting great speeds: ~300Mbps symmetrical up/down. My Mac Pro (wired) is getting great download ~350Mbps, but terrible upload speeds (~60Mbps). Using speedtest.net, I noticed that the speed starts increasing from the start (~20 Mbps) to about 1/10th of the way through the test (~40 Mbps), then chokes, drops back down, then slowly increases until the test stops around 60Mbps. I have tried deleting the VPN profile and recreating it. This speed difference is very important, as I am shuttling massive amounts of data (hundreds of gigabytes) across this connection.
What could be causing this and what can I do to get full upload speeds out of my client?