Doing an export of a number of users causes high CPU and I/O usage. I can mitigate the effects of the CPU usage using Resource Manager and setting the export session to a low priority group, but how can I limit the disk I/O? I am not running the operation in parallel and have already read Oracle's Datapump Performance documentation. Is there anything else I can do?
Ionice can limit disk usage
Take a look at the ionice utility for linux, as it seems to suit your needs quite well.