You are at pretty normal speeds for PPPoE using a Dream Machine Pro/SE due to slow CPU speed in them (1.5ghz on most models and using an architecture from 2012). PPPoE uses extra CPU processing for the overhead of the protocol, and while there are software tweaks manufacturers could do to increase PPPoE performance, there is nothing you can do on that front, and I have no idea if Ubiquiti has done those tweaks or not already anyway.
https://community.ui.com/questions/What-is-the-max-performance-for-PPPOE-on-UDM-Pro-With-Solution/67057f47-509e-4f8b-8edd-5dc29f380759
The only way to increase it is to move to a different Gateway that has higher speed capability. For instance the Firewalla Gold Pro achieves 9gbps down and 7gbps up:
Firewalla Gold Pro: 10G Cyber Security Firewall & Router Protecting Yo | Firewalla…
EDIT: The Unifi UCG-Fiber is newly released and confirmed to have hardware NAT that accelerates pppoe connections. Tests have shown it gets 8dn/6up PPPoE speeds. So Unifi now has a gateway that is fast for those connects, and costs very little. A much better deal than the Firewalla.