PowerShell Script to delete all Temp Internet Files, All Users

Temp. internet files are excluded from roaming profiles by default, are you sure that's what's slowing things down?  By default, temp internet files are saved to this path in the "appdata\local" folder, which is not part of the roaming profile normally:
C:\Users\%username%\AppData\Local\Microsoft\Windows\Temporary Internet Files
or
%localappdata%\Microsoft\Windows\Temporary Internet Files

See this page:
http://stealthpuppy.com/reduce-logon-times-by-excluding-the-bloat/

I would look at cookies. Even though each cookie file is a tiny text file, if you have 10,000 of them, it's going to slow down things during log on and log off.
The cookies folder IS part of the roaming profile by default:
C:\Users\%username%\AppData\Roaming\Microsoft\Windows\Cookies

We used to redirect Cookies to the user's network drive so this wouldn't be an issue.  At my old job on Server 2003 servers, the users' H: drives held all of the redirected folders: Favorites, Cookies, AppData, My Documents, My Pictures, etc.

You can have the user's clear cookies using Internet Options and then have them log off & back on to see if that helps. If it does, I think you found the culprit.