try another outlet (and/or outlet country)
that's what i do when my vpn client grows sluggish.
Coming back to this issue, unresolved after over a year. Blaming the VPN or the outlet doesn't make sense when the slowing down only happens on router vpn. Desktop/client VPNs maintain full speed indefinitely. Also blaming VPN speeds makes no sense when a fresh reset router can ping 10ms and get 300Mbps without breaking a sweat.
Something about OpenVPN on AC86u is causing the VPN to slow down to a crawl after several hours, from 300Mbps to less then 1Mbps. I've done factory resets, etc. The exact same OVPN config file can maintain over 100Mbps on a cheap Android device.