Reminder: this is from a Linksys E8450 (UBI) MediaTek MT7622BV (Dual Core A53, 1.35 GHz) running OpenWrt 23.05.2. The benchmark in the original post gives 302 Mbps without -R and 310 Mbps with -R.
Yesterday, I purchased the OVPN subscription with a static public IP (the last one available in Singapore from them), and unfortunately, the benchmark does not reflect the real-world speed achieved. Please take this into account when interpreting others' results.
The test above was done via WiFi from a laptop, with the VPN configured on the router.
The underlying connection is symmetric 300/300 Mbps PPPoE on paper, 350 Mbps in reality. However, the speed achieved is lower than the output of the proposed benchmark (302 Mbps). The router's CPU is indeed a bottleneck. Perhaps this is due to the PPPoE and WiFi overhead.
For comparison, here is the speed test result from the same laptop without the VPN:
And with the VPN configured on the laptop instead of the router: