Like coils, all capacitors represent an energy storage.
When a capacitor is connected in parallel to a loudspeaker something else
happens. If the voltage to the loudspeaker needs to change, the capacitor
must be recharged first. The capacitor prevents quick voltage changes, therefore
passing on only low frequencies to the loudspeaker.
Therefore: lossy capacitors don't let the current pass through unhindered
at high frequencies since the terminals represent an ohmic resistance. Besides,
they are not insulating at lower frequencies, because the insulator shows
a measurable conductance.
Additional important notes: If a crossover is optimised for a low quality
capacitor, the use of a high quality capacitor often doesn't show any advantage.
The crossover needs to be tuned again incorporating the high quality component.