Yes, LPG tends to down vehicle's performance by a small percentage. But not that much effect did on engine performance. We all know LPG has lower calorific value than petrol so LPG gives less potent as an internal combustion engine as a fuel.
On the other hand, LPG is less than half of the price than petrol and it has a high octane rating than petrol so it is more easily mixed with air. So that reason LPG is also much helpful in such case but after all, if you use LPG in your car as fuel your car maintenance increases and as time goes some part of the car will need frequent replacement.