You can test maximum bandwidth between the two points with iperf. As other mentioned, lowering the encryption algorithm used could improve the performance. You could test the performance of your hardware with openssl speed -evp aes-128-cbc for example. Another issue with Linux ipsec is that it binds the encryption/decryption workload to the softirq of the ethernet card, so you cannot take advantage of multi-core cpus unless you use multiple tunnels and network card with multiple queues.
My Atom D525 1800MHz router can do 28MB/sec in openssl speed -evp aes-128-cbc while it can do 52MB/sec on aes-128-gcm. On the other hand, My VU+ Duo2 can do 10MB/sec on aes-128-gcm but 21MB/sec on aes-128-cbc. So choosing the crypto algo is important depending on what hardware you have.
If you want maximum IPsec AES performance, the new Atom C2550 is a great chipset, since it supports AES-NI. More than 400MB/sec in aes-128-cbc