One of our clients is experiencing this issue in SL 2018 CU1. They also experienced the same back in 2011 which they just upgraded from. So it's not a new issue.
Many of their projects have a lot of tasks, hundreds. It would take 1 minute to load such a project into Project Maintenance (PA.PRJ.00) screen, from the time that the project ID is keyed in, to the time the data is displayed. While loading, the screen is not responding.
I could confirm that the speed is directly caused by the number of tasks, as the loading time improves with the reduction of tasks linked to the project.
Is this a known issue that will be addressed? Is there a workaround?