Engine performance of convertible

I would say weight but I may be wrong

It would also depend on the engine, gearing and transmission.

The aerodynamics also come into play. Stick shift or automatic. Size of tires and type, tune- up or has been running long. high octane gas or regular? Driver, gearing, weather or time of day. Hotter temp runs slower. Slower than what? In traffic most cars all go nearly the same speed.