The MPEP (§708.02(a)) requirements are at least intended to assure that prioritized examination should have minimal impact on other examinations.
Basically, prioritized examination moves a great deal of the burden from the examiner to the petitioner. To give some specifics:
- The application must be filed electronically.
- The application is restricted to no more than 3 independent claims and 20 claims total.
- There can't be any multiple-dependent claims.
- The applicant can't argue patentability of dependent claims separately during any appeals.
- The petitioner has to agree to prior-art interview at the convenience of the examiner.
- The petitioner (rather than the examiner) has to conduct a full prior art search including US and foreign patents, applications, non-patent documents, has to specify the field of the search, etc. In other words, the petitioner has to satisfy the examiner that they did at least as good of a prior art search as the examiner would normally have done in a normal application.
- The petitioner has to summarize that prior art search in an Information Disclosure Statement that directs the examiner to what they believe are the best references for each claimed feature, how the current invention is distinguished from each reference, etc.
So, while I haven't found any solid references about the outcome, the rules are clearly intended to prevent priority applications from delaying other applications, primarily by shifting the majority of responsibility for examination to the petitioner.