AWS site-to-site VPN poor connection

I recommend conducting iperf tests using both the private IP address (inside the tunnel) and the public IP address peer (outside the tunnel). Comparing results from multiple test configurations will indicate whether the constraint is within the VPC connection or is limited by overall internet capacity. Try adjusting parameters like using multiple streams and modifying the window size.

If your internet egress is multi-homed, run a bi-directional MTR test to the public IP peer to analyse the network path between AWS and your gateway. Based on the MTR results, you could try modifying how your prefix is advertised using BGP attributes to change the preferred network path. Taking these steps will provide insight into optimising performance.

Hello - below might be helpful

https://aws.amazon.com/blogs/networking-and-content-delivery/aws-site-to-site-vpn-choosing-the-right-options-to-optimize-performance/