(a) Frederick D., Mohler J., Vorvoreanu M., Glotzbach R. (2015) The effects of parallax scrolling on user experience in web design, Journal of Usability Studies, 10 (2), 87-95: http://uxpajournal.org/the-effects-of-parallax-scrolling-on-user-experience-in-web-design/
Excerpt from article abstract:
Participants believed that the parallax scrolling website was more fun than the non-parallax scrolling website. The results of the study also showed parallax scrolling to be more effective when used in a hedonic and fun context. In spite of these benefits two of the participants suffered motion sickness and experienced significant usability issues while interacting with the parallax scrolling website. As a result, this potential risk to participants raises some ethical issues that UX practitioners and web designers should consider when planning to implement parallax scrolling.
(b) Parallax Scrolling: Attention Getter or Headache?: http://www.uxmatters.com/mt/archives/2014/11/parallax-scrolling-attention-getter-or-headache.php
With few exceptions, parallax scrolling is just a flashy gimmick from a user experience perspective. Its primary usefulness is to show off how cools a designer is... The pro-parallax camp feels that the stunning visual nature of this effect improves the user experience. Parallax can guide and delight users. On the other hand, the anti-parallax camp feels that parallax scrolling stunts the effectiveness of Web sites... Although this article may seem to be a scathing denunciation of the usefulness of parallax scrolling, it’s not all bad news for parallax lovers. If you use parallax in moderation and stay within the bounds of these sophisticated optimization strategies, parallax scrolling actually has the potential to delight users, thus improving a Web site’s user experience.