Long installation times for SharePoint cumulative updates are nothing new. We have already dealt with a similar problem in SharePoint 2013, see Speeding up CU and SP installation on SharePoint 2013. In SharePoint Server Subscription Edition (SPSE), we can also reduce installation time by stopping certain services before starting the update.
This time, however, the cause is different. In SharePoint 2013, one of the issues we addressed was the resources consumed by running services. In SPSE, the mechanism used by the update installer to stop SharePoint and IIS services does not work as expected. The services remain running and use files that the installer is trying to update in the Global Assembly Cache (GAC). This makes the installation take longer.
So, if you expected that moving to a newer SharePoint version would mean you no longer needed this procedure, unfortunately that is not the case. The solution is to stop the required services manually before installing the CU and start them again once the installation has completed successfully.