Most of the speed problems are Outlook issues and can be resolved a number of different ways. Before I begin, let me explain the difference between a .pst and an .ost. If you are using outlook to download e-mail from your e-mail provider via pop3, IMAP or some other service, you are most likely storing your e-mail in a .pst (personal store). If you connect to an exchange server to get your mail, you most likely running in cached exchange mode and have an .ost. The .ost is just a cached version of what is stored for you on the exchange server but is located on your hard drive similar to a .pst. Now that that is out of the way, lets move on to the actual tweaks.
1) The enhancement that I find gives me the most bang for the buck (this is for Outlook 2007 users) is to rename the extend.dat file under the individual user's profile. On XP this is located in c:\documents and settings\
2) The way I understand it, Outlook loads its data store into memory at startup. If you have a lot of e-mail, this can decrease the performance of Outlook significantly. Archiving e-mail into a separate .pst file or deleting old e-mail can help performance.
3) Prior to outlook 2003, .pst's and ost's could not exceed 2GB's. In 2003 and later this issue was fixed but for many users, outlook performance decreases significantly when the .pst and .ost sizes reach the 2GB and greater mark. If your .ost file is larger than 2GB, try archiving old e-mail into a local store (.pst). OST's do not shrink in size once data has been removed so if you recently deleted a large quantity of mail and noticed that your .ost hasn't shrunk in size, you can fix this by renaming the .ost file (with outlook closed). When outlook opens again it will connect to the exchange server and re-create the .ost file using a more appropriate size.