First of all you are talking about GVM and not OpenVAS. openvas is only the scanner component nowadays. Therefore you should also use apt install gvm instead of apt install openvas. Technically both commands are the same because the nice Kali packagers added a provides openvas statement to the gvm meta package.
Second GSA is a web application written in JavaScript and running in your browser. It communicates with the gsad web server that communicates with gvmd. gvmd is responsible to the stuff you are talking about. Please take a look at About GVM 20.08 and 21.04 Architecture
After a file sync of the feed the data needs to be loaded by gvmd into the postgres db. The feed sync itself takes some time depending on the network connection and available bandwidth. After the data is synced gvmd needs to parse this data and put it into the database. With the next feed sync only a diff is loaded and therefore the whole process is much faster. But for the initial first sync this just needs some time.