How to delete 1600 users and 2500 posts?
You can use Views Bulk Operations, a module that allows to execute bulk operations on nodes, users, comments; it also permits to apply the selected operation to all the nodes, users, or comments using the Batch API. The batch API allows forms processing to be spread out over several page requests, thus ensuring that the processing does not get interrupted because of a PHP timeout, while allowing the user to receive feedback on the progress of the ongoing operations.