There are so many factors that contribute to scan time. It’s really a combination of the target server(s) response, scanner host system resources, configuration settings, and perhaps other factors. Modification to the openvas.conf configuration file could help speed your scans. You can modify the max_hosts, optimize_test, time_between_request, open_sock_max_attempts and timeout_retry to optimize the scan config, lower the tested port range with port_range, and increase max_sysload to use more CPU power on the scanner host, but this will not account for a slow response from the target.
I can also add that limiting the port_range from the web-interface should reduce the scan time, but I’m not sure it that alone would achieve the increase you are looking for.
My best guess to optimize the scan would be run thin server and interact with openvas-scanner directly via the command line. That way, you can have more control.
For example, you could manually filter the .nasl files (NVTs) by CVSS and run openvas-nasl from the command line as outlined in this post.