A user has been migrated from a legacy desktop environment to the company's new Citrix...

Comprehensive and Detailed Explanation From Exact Extract of Nutanix End User Computing documents:

According to Citrix on Nutanix Best Practices for Profile Management:

Legacy Roaming Profiles: A "Legacy Roaming Profile" (Windows Roaming User Profiles) functions by downloading the entire profile (files, folders, AppData, etc.) from the network share to the local VM at logon, and uploading changes back at logoff.

Impact on Performance: If a user has a large existing profile (common in legacy migrations), this copy process creates massive network and disk I/O activity during the logon/logoff phase, resulting in extended wait times (the "Applying User Settings" or "Waiting for User Profile Service" screens).

Contrast with Modern Solutions: Modern VDI solutions use Profile Containers (like FSLogix or Citrix Profile Management containers) which mount the profile as a virtual disk (VHDX) almost instantly, rather than copying files.

Why not Home Folder (C)? A home folder is typically a mapped network drive. It does not copy data to the desktop at logon; it simply maps a pointer. Therefore, having "too much data" in a mapped drive does not slow down the logon process itself.