How did this person perform last year in the same period? How did the person perform in, say, January - March?

Do you have an HR department? This is absolutely what they're there to help you with.

"Fasting, not drinking through the warm days, and being sleep deprived has not helped his attention to detail." - You don't know the reasons for "poor attention to detail" so stop speculating. Wait until Ramadan is well over then have a conversation. Focus on the results only. None of that is your problem, nor is it your business. As @PhilipKendall points out - you should work with HR on this. They are trained in these situations.

@EdwinLambregts: Impossible to compare his performance to last year. He's been here for 6 months.

Well, Ramadan is over this weekend (or monday for some). I don't think addressing it now wil add any value if he will only fast for one more day imo.

There's been continued performance problems you've already discussed with him previously. It's more discriminatory to not continue that conversation because he's observing Ramadan (because that would be you letting his religious beliefs interfere with your professional interaction).

Replace your second paragraph with this one: "My line report recently went vegan; the lack of protein has not helped their attention to detail". Do you see the problem?

@TasosPapastylianou I think that's a poor analogy. Vegans can still get complete, proper nutrition and hydration. Intentionally abstaining from all food and water is quite a different issue.

@alroc I'm sure a lot of, if not most, people observing Ramadan keep up their work performance just fine. Either can be done poorly or can be done well and can affect some extremely negatively while not affecting others at all. It's not exactly the same, but there are similarities.

@alroc it's what they claim to believe. Ramadan advocates would suggest the opposite.

@alroc alas, I was waiting for someone to miss the point. I believe this demonstrates a similar bias blind spot as the OP (which is fine). The problem isn't one of nutrition, it's one of discrimination over a lifestyle not shared / understood by the OP. Furthermore, ironically, you are absolutely right, yet the same can be said about fasting: there is overwhelming evidence fasting improves mental acuity and productivity, i.e. the opposite of what OP (and presumably you) might expect. So, if anything, it was a good analogy. But if one still objects, try also "married a chinese wife / noodles"

@alroc furthermore, it was not meant as an analogy, but as a generalisation. The fact it was perceived to be an analogy is part of the issue here (i.e. differentiating between the two as one being consistent / famliar with own beliefs vs not). Form a professional / HR perspective It's exactly the same problem on paper. That is the point here. Not the specifics of the discrimination.

@TasosPapastylianou: "The productivity of workers declines in the holy month by 35-50% as a result of shorter working hours and the change in behaviour during this month". Source: bbc.co.uk/news/business-19281749

@WorkerWithoutACause This is a good question but the title is confusing; you are concerned with the worker's habits, not their beliefs. Consider changing the title to something closer to "How do I address poor performance when religious practices could be a factor?". The current title suggests that you blame the employee's religion, and it draws responses that don't answer your question.

The same way you'd otherwise address poor performance.

@TasosPapastylianou I think a better analogy would be something like My line report has consistently underperformed, and recently there has been an even greater drop in productivity. This drop happens to correspond to a significant change in diet. How do I address this lack of productivity without looking like I'm also addressing the diet? (Of course, it would have been better if the OP had worded the question so that it was explicitly about correlation, rather than causation.)

@Tasos Papastylianou: The base problem here, if I'm reading the question correctly, is not the different lifestyle, it's the ongoing lack of productivity. It really shouldn't matter WHY that lack exists, but the OP has a theory and so fears that addressing the problem would lead to messy charges of religious discrimination.

"Fasting" during Ramadan should not be construed as poor nutrition. They have their meals at night during that time. Maybe they eat differently, I don't know. But the use of the word "fasting" misleads those who don't know the context.

@WorkerWithoutACause depends on the source (I have also seen reverse figures), but even so, I feel it's irrelevant. Female employees demonstrate "a change in behaviour" on certain days a month which potentially "affects their performance" as well. Would you have flagged that worker's reluctance to use contraceptives as being a productivity issue? No, right? This is the same issue. Your focus should be the performance itself and the steps the employee is taking to address them, not what you think may be the underlying cause behind them in terms of their lifestyle, biology or private beliefs.

If you can't discuss it in a bar, you shouldn't discuss it at work.