Small tick and loss of power

I tend to believe the top of the engine having a problem but a small leak at the cat won't cause a power loss

Sounds like a top end issue with either lifters, valves, rods, etc. Minor "ticks" are not serious but if it gets progressively louder, you may want to have it addressed by a reputable mechanic. The higher the mileage of the car, the more likely that things like this will develop.