At 60Hz the reactance of the 200pF capacitor is 1/(2*pi*60Hz*200pF) = 13.26 megohms. As chemelec mentioned, it has little effect in the sense that it is effectively a high value resistor across the output of the transformer. However, the capacitor will charge up until the voltage is high enough to jump the spark gap (at 200pF I would expect it to charge up rather quickly, depending on the NST current), at which point all of the stored energy will be released at once across the gap. This is what makes the spark so intense. It will do this at 120Hz with a 60Hz mains input, or 100Hz with a 50Hz mains input.