It is extremely likely that (1) your "research" approach is considered antisocial, and (2) you are so completely misunderstanding both the ODP and itself that you are churning results unnecessarily.
Rather than machine-gun searches, you should learn how to use the categories, and the links between them, to get a grasp of the totality of information the directory presents on a subject. The ODP search itself, restricted as it is to words contained in the URL-title-description, is unlikely to give a fair impression of the wealth of material available. After it points you to the right categories, it's time to switch over into "browse" mode, which obviously doesn't stress the servers so much.
Yet another alternative is the Google "directory search", which is neither so resource-constrained, nor is it so keyword-constrained.
One final possibility: if the purpose of the research is to learn how manipulate ODP search results, then the activity, besides being maliciously antisocial, is futile in two different ways: (1) the order of search results is an arbitrary random order, not any manipulable "page relevance" calculation, therefore even editors cannot affect it, and (2) the ODP site search is not that heavily used by surfers (most of whom are better served by, say, Google directory search): therefore result order really won't affect a site's visitor statistics. In this case, one would be well advised to focus research elsewhere.