That's normal. The A/C compressor is attached to the engine with an accessory belt like your power steering and alternator but it uses a clutch that can disengage the pulley on the compressor when your not using your A/C. In order for an A/C system to work, the compressor has a considerable amount of work to do so it does take some power from the engine. Some newer cars use electric pumps to run the A/C system (and even PS), I don't know if the newer Civic do or not though? Some cars, especially those with larger motors and therefore more torque, the effect is less noticable.