Thank you for your question. What is ‘normal’ in terms of access speed is difficult to say, given so many factors may affect address book access. The fundamental difference between the System and Personal address books is that the System book is a shared resource by default, whereas personal books are not. This difference can affect the responsivness of programmatic access to address books (and any shared resource). Factors include your network topography, GroupWise system design, load and infrastructure.
Depending on your specific needs, you might be able to create a personal address book based on the system address book for some purposes. This is obviously not ideal, but it is an alternative in some situations.
What sort of difference are you experiencing accessing shared v. personal address books? What is the performance like if you create a personal book containing the same number of contacts as your shared book?
I hope this information is of some assistance.