Shell load time covers alot of situations. What I like to do is set  the registry key

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]

"DelayedDesktopSwitchTimeout"=dword:00000002

that was mentioned before to a very low or zero value. This basically removes the preparing windows screen, why that helps is you may be able to see whats going on behind it. I found an outdated logon script eating up 8 seconds of my logon time doing that. Also set the "display highly detailed status messages." gpo setting to see how long each service takes.

https://www.thewindowsclub.com/enable-verbose-status-message-windows

past that if you use DEM, there are more performance tools that may help find slow areas there. One thing to keep an eye on too is cpu usuage, I know for me with 2 cpus a good part of the login is 100%, if you have alot going on during startup and are trying to use 1 cpu, its really not enough. I'm getting around 30-40 seconds with 2 appstacks being attached using 2 cpus and 8gb of memory on 1809