Capacitors are never 100% capacitance. Their characteristics include resistance and inductance.
Capacitors are too often taken for granted. When it comes to high-speed circuits, capacitors can behave like resistors or even inductors, depending on the frequencies involved. When an IC needs power, decoupling capacitors can help provide it, but not always.
In theory, you can increase capacitance in a circuit by placing capacitors in parallel, right? See how much you know about how capacitors behave when the frequencies are high. As always, answers appear at the end.