Technically, the snorkel should first intake in the clean air higher up. In many vehicles, the air intake is located somewhere where it is dusty. Only if the intake is really waterproof, the snorkel might help at deep water crossings. However, this is not the case with many standard solutions, because somewhere plastic moldings are often simply plugged into each other. So you have to build or modify something indvidual to make it waterproof. Please keep in mind, however, that not only modifications with regard to the air intake may be necessary here. For example, axle and transmission breather or the location of any electronics must also be considered with regard to water crossings.
Of course, a snorkel should not prevent you from checking and changing the air filter regularly, if this is planned or necessary in the maintenance schedule.
Whether the snorkel reduces performance depends on its design, length and diameter. A very individual thing and also depends on the vehicle. This is a question that cannot be answered in a general way.
By the way, if you want a snorkel, I would say: mount one!