I was able to fix Outlook slowness issue while accessing the Public Folder by making the registry changes on Public Folder Server, as Outlook client were trying to connect to Public Folder Server which were in US.
I followed this article and applied Method 2 to fix the issue.
But, the issue at US location was Outlook client were prompting for credential when trying to connect Public Folder Database which was not a Default Public Folder Database on Mailbox Database. So to fix this issue I had to apply the same registry fixes but I also had to install Hot Fix on Public Folder Server. Hot Fix is part of Method 1, but this article doesn’t have a link to download the Hot Fix.
I had to call Microsoft get this Hot Fix which you can download from here. Writing this article, so that you don’t have to call Microsoft to get this Hot Fix.
I hope it helps.