After you press the 'Start' button, PerfectIt needs to gather lots of information about your document. That can take time. The bad news is that the longer your document is, the longer PerfectIt will take to gather information. The good news is that once the initial gathering of information is complete, PerfectIt runs all of the tests much faster.
It's worth understanding that PerfectIt works by running hundreds of searches per second. The longer a document is, the longer each search takes. Also, the longer a document is, the more searches PerfectIt will have to run. This is especially the case with documents over 200 pages. With shorter documents, PerfectIt will run faster.
If you're using PerfectIt Cloud, you can significantly increase the speed of PerfectIt by reducing the filesize of your document. One of the best ways to do that is to compress images in the document and then save it. Instructions for that are here.