Microsoft says Outlook might take a minute to start and display the splash screen on Windows 10, version 1809 or later if User Experience Virtualization (UE-V) is enabled. The bug is caused by the UE-V Sync method being set to None which is causing Windows to ignore the default synchronize timeout of two seconds. Microsoft recommends switching the Sync method to the default Sync provider. The other workaround requires customizing the template to change the behavior of the default template. Microsoft reminds Outlook users to not edit the template for a version of Office 2019 or 2016 since it will potentially cause crashes.
Source: https://www.bleepingcomputer.com/news/microsoft/outlook-affected-by-one-minute-startup-delays-on-windows-10/