When doing a full and fast scan using 1 Maximum concurrently executed NVTs per host, scan is completed between 8-12 minutes. The status of the scan also moves (eg. 10%, 32%, 52%, etc.) until completion.
While using 4 Maximum concurrently executed NVTs per host, the scan takes more than an hour to complete and it shows just 1% as progress. Then it will jump to 50%~92% after an hour.
OpenVAS9 is installed in Ubuntu 16.04 Xenial with the following versions.
openvas9-manager - 7.0.3-1
openvas9-scanner - 5.1.2-1
greenbone-security-assistant9 - 7.0.3-1xenial2
Same behavior is observed with OpenVAS9 using SQLite and OpenVAS9 using PostgreSQL as database.