The following commands can help you test the performance of your CPU under OpenWrt, especially the encryption and decryption performance, and can provide a performance reference when you use VPN.

AES:                 openssl speed -evp aes-256-gcm
ChaCha20-poly1305:   openssl speed -evp chacha20-poly1305
Coremark:            /etc/coremark.sh && cat /etc/bench.log

Below is the benchmark score of my OpenWrt CPU: Intel N4200.