Is working in month of Ramadan affecting employees performance?

Yes it does affect employees performance, BUT only for that month

For Muslim colleagues physically , emotionally and psychologically it does but only for the month of Ramadan. Study shows that during the month of Ramadan Many Muslim colleagues are affected due to lack of sleep and food fasting  (physical) and due to this people tend to be have mood swing (emotional) and cannot  think clearer, therefore, it affect employees performance as well as the company entire productivity. But if you mean the annual employee performance I don't think so since it only a month of so as compare with the entire year to be evaluated.  My two cent view.

I think the hardworker will not think that Ramadan will affect his work

It should not. When you´re fasting, you cannot work long hours butyou can perform more efficiently. I used to deliver more while I´m fasting.