Given lack of testing and that many countries (including European countries) are seeing Covid surges right now, I think it's highly likely that most competitors are competing with either current or recent Covid infections affecting their peak performance capacity.
Edit: I wouldn't suggest it's the sole cause of performance issues. But for an entire article on the topic of swim performance to completely ignore multiple reports of viral infection from top performers seems a glaring omission.
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1: "The 29-year-old does not have to isolate from other athletes and does not have to test negative before competing again": https://www.espn.com/olympics/story/_/id/40672610/olympics-2...
2: "For now, nothing has been put into place by the organizing committee … but hand sanitizer is available in its clinics and restaurants.": https://www.cbsnews.com/news/2024-paris-olympics-covid-cases...