Politeness
crawler4j is designed very efficiently and has the ability to crawl domains very fast (e.g., it has been able to crawl 200 Wikipedia pages per second). However, since this is against crawling policies and puts huge load on servers (and they might block you!), since version 1.3, by default crawler4j waits at least 200 milliseconds between requests. However, this parameter can be tuned:

crawlConfig.setPolitenessDelay(politenessDelay);

 protected CrawlController createCrawlController(String crawlerName, CrawlerConfig config) throws Exception {
        CrawlConfig crawlConfig = new CrawlConfig();
        crawlConfig.setCrawlStorageFolder(Files.createTempDirectory(FileSystems.getDefault().getPath(Path.getTempDirectory().getAbsolutePath()), crawlerName).toString());
        crawlConfig.setMaxDepthOfCrawling(config.getDepth());
        crawlConfig.setMaxOutgoingLinksToFollow(config.getMaxOutgoingLinksToFollow());

        AuthInfo authInfo = createAuthInfo(crawlerName, config);
        if (authInfo != null) {
            crawlConfig.addAuthInfo(authInfo);
        }

        PageFetcher pageFetcher = new PageFetcher(crawlConfig);
        RobotstxtConfig robotstxtConfig = new RobotstxtConfig();
        RobotstxtServer robotstxtServer = new RobotstxtServer(robotstxtConfig, pageFetcher);

        CrawlController controller = new CrawlController(crawlConfig, pageFetcher, robotstxtServer);
        for (Site site : config.getSites()) {
            log.debug("Crawling site: " + site.getUrl());
            controller.addSeed(site.getUrl());
        }
        return controller;
    }