When Directory Server reads or writes data, it works with fixed blocks of data, called pages. By increasing the page size you increase the size of the block that is read or written in one disk operation.
The page size is related to the size of entries and is a critical element of performance. If you know that the average size of your entries is greater than db-page-size/4–24 (24 is the per page binary tree internal structure), you must increase the database page size. The database page size should also match the file system disk block size.