Hi,
Can you confirm the query http://:9000/api/rules/search?xxxx of your first screenshot is slow to respond when you try to open it in a web browser?
If yes, then the issue is on server side. We might need your SonarQube server version + DB provider + DB version.
Also you have to ensure your server specs (both SQ server and DB) are large enough (look at CPU/memory usage when calling the WS).