We have a workflow requirement on shared workstations for users to run the SfB client as their own account. The workstation is signed in as a generic AD account. When the user runs SfB for the first time "as themselves" (i.e. shift-right-click, run as different user), there is a significant delay in signing in, sometimes up to two minutes. After the first login, each subsequent login takes a normal amount of time. I can repro when I remove the run-as user's profile.
Starting the client as normal and providing the credentials of the user is not an option. The client must be run as the user's AD account.
Run-as also uses the Internet settings for the run-as user. We found that the run-as user wasn't getting the right Internet settings (in this case, WPAD) that a user would get if they logged into the PC. So we added those settings to the app we use to run-as and it fixed the issue.