Large PST's and numerous PST's can surely slow down Outlook.
I create a monthly archive PST in which I manually store my important read and sent emails and I delete the unimportant emails as soon as I can. At the end of the month, I disconnect the monthly archive and start a new one.
I use X1, a powerful search tool to search my closed archives. X1 allows for more granular searches and does a much faster job at it than Outlook does.
This method keeps Outlook's open ost and pst's to a minimum and helps Outlook open up pretty quickly.
Of course, if you have an email journaling tool on the network, there should be little need for user based email archiving. Users can simply delete everything they read every month and use the journaling search tool Outlook plug in to find old emails.