Fasting doesn't impact performance once you get used to it. I fast for around 20 hours ever single day and I'm fine. For men, give them about 3 days to a week to adjust. For women it should take about 2 days to a week. These are typically the time frames people expect to stop feeling hungry when drinking nothing but water. Since religious fasting holidays typically permit eating at some point or in some measure, most people won't actually be lacking any nutrition so there won't be malnourishment going on. At most there should be some mild hunger.
Keep in mind, that as an employer you're actually totally powerless when it comes to ensuring people eat. If their health isn't negatively impacted by fasting - it probably won't be - you actually can't do anything about it at all.
That being said, you can do something about a dip in performance. I'd treat this like that. When you find out the reason is a fasting based religious holiday, ask if they can drink water. If they can, suggest they drink that. For the rest of it, just explain that their performance needs to be about what it normally is. Explain that fasting is not an excuse for performing at less than 80% for more than a week. This is literally all you can do aside from forcing them to take lunch breaks anyway. Do note that no one actually has to eat on their lunch breaks.