Gigabit speeds are supported by the AmpliFi router, however, there is an option called Hardware NAT that needs to be enabled in order to achieve those speeds. Navigate to this article to understand what it is and how to enable it: Enabling Hardware NAT. (AmpliFi Instant and Alien do not have hardware NAT)
If enabling Hardware NAT does not fix the issue, please take a look at this article to troubleshoot other issues that may be affecting speed: Troubleshooting: Slow Speeds.