If you are using DEM my guess is that you are using non persistent machines, right?
In the Windows is preparing part of the logon process it normally is one of 2 things that cause this,.
1. A lot of GPO settings that are being applied, mostly preference mode policies. If possible only set machine GPO's in Windows policies and all other policies and user settings should be done by DEM.
2. Do you happen to have a lot of W10 modern apps? What do you see when you type in the command get-appxprovisonedpackage. When logging in for the first time (which is every time if you are using a non persistent machine) all windows modern apps are being installed into the profile.
Looking at DEM, you could check the logon monitor or the log file itself (if you create one during logon) to see how long DEM is doing actions. These actions normally are being done before the preparing windows. Also you could try and enable verbose logging during logon.