Defrag File System for Drive containing PST/OST files
After Compacting PST and OST files, Defrag the file system containing the files. I have seen some incredible improvements by doing this, especially on older computers.
Also, Compact regularly, especially OST files. I have seen many OST files that are four times larger than they need to be.
By regularly compacting, you stop this happening and also avoid a Compact operation that could take hours.