Simply put resource degradation is typically the reason... but other thing can be in the mix, like secondary storage or errors.
As a first step, I would suggest a glance at the EV Event viewer. It can often tell you where the issue is.
The only way to really be able to tell is to DTrace the processes and analyze the logs to see where the degradation is. These days most of the functions in the logs have a time taken stamp to search for... and that can help you identify the source of degradation.
A few things I would suggest to keep in mind. This may be the first point of contention, and your journey to expeditious native export may find many bumps on the path to consistent performance. Also, if you are seeking to do this at scale, you would be wise to consider using a third party migration utility like (shameless plug) Archive Shuttle to export your content. Native tooling was not really made for quick and accountable mass export.
