With latest SQ 9.9.1 some performance issues are fixed:
https://community.sonarsource.com/t/sonarqube-9-9-1-lts-released/90049

• Computing the total number of lines of code is slow on large datasets https://sonarsource.atlassian.net/browse/SONAR-18872?jql=project%20%3D%2010139%20AND%20fixVersion%20%3D%2014051
• Telemetry performance issue https://sonarsource.atlassian.net/browse/SONAR-18472?jql=project%20%3D%2010139%20AND%20fixVersion%20%3D%2014051

If scanning is slower after upgrading to SQ 9.9 LTS, first try upgrading to SQ 9.9.1 to see if it solves the problem. Use also the latest cxx plugin 2.1.1 or newer.