I spent the better part of a year trying to resolve my Citrix login issues. Our Citrix servers were not overly taxed, running on 4yr old Cisco C220's who's resource consumption is less than half. We upgraded our storage to a NetApp A200 and still had login issues. I mention this just to point out that the issues did not point to being hardware or resources.
I scoured blogs and posts about optimizing our Citrix image. Using the Citrix and VMware optimization tools, turning off all non-essential services, reg tweaks, starting with a fresh image to ensure changes were causing issues, etc. The issue always seem to be loading the roaming part of the Windows profile. The static folders: Desktop, Documents, etc. loaded just fine, but if someone had an issue logging in deleting the cache folder of their settings would "fix" the issue and they could log in and setup their profile again.
The one thing that actually fixed all my issues was FSLogix! Not only did it fix these issue, but login times went from 1-5min to 30sec-1min max. I have had zero issues with logins since and the setup was very easy.
Many of my IT friends that manage Citrix have never heard of FSLogix, so I felt it was appropriate to give an endorsement to a great product. Plus if you have an EA with MS you are already licensed to use it.
https://docs.microsoft.com/en-us/fslogix/overview