Compact the OST as well or recreate it.
The cached Exchange mode can lead to large ost files. These can be compacted in the same way as above. Check your total folder size against the size of the OST file (which I usually find using %temp%\..\Microsoft\Outlook to reduce time to browse to it). If the OST is more than a few 100MB larger than the folder size, it can be quicker to close outlook, delete the OST and then reopen Outlook to reload the data from the server (depending on speed of network) as niko88 mentioned. I had one client with a 19GB OST file when his server data was only 2.5GB and taking 10 minutes to open Outlook. Dropped to less than 20 secs after deleting and reloading OST.
Hope this helps.
