I am in the process of migrating from L2TP/IPSec to IKEv2 VPN and am simultaneously addressing the issue of VPN traffic being significantly slower than direct traffic.

Test method: Downloading a single 2GB file with curl from a third-party server.

Router: RB1100AHx2
OS Version: 7.14.1

Download speed from the office (server behind the router): 80MB/s (wired connection)

Macos Sonoma client outside office (wifi)

Direct download (no vpn): 40MB/s
with L2TP/IPSec connection: 2MB/s
with IKEv2 connection: 1.8MB/s

The router's CPU load, according to /system/resource, is around 20%.

Configuration below: