Web ui/api unresponsive during PersistLoadProcessor preload, which can take a long time. This includes the engine page and job page of the starting job. Reason is that PersistLoadProcessor preload happens inside of CrawlJob.startContext(). Requests to these web ui/api urls invoke CrawlJob.getCrawlController(). Both methods are synchronized (on the CrawlJob object). Here's the commit that synchronized the methods:
Not sure what the effect of removing synchronized on getCrawlController() would be. Another option would be to do the preload at a different time.