You are talking about an abstract case. Yes, if an objects lifetime is perfectly easy to determine, manual memory management is faster. But in many typical programs with real dynamic memory requirements, this is not the case. One has to keep track about the life time of objects in the program logic. Either by storing additional information as for reference counting, or careful designing your program logic so the status of an object is always clearly known.
The latter might not bear a cost in CPU cycles, but certainly in programmer cycles, if the program logic allows for it at all. It also puts the burden of correctness on the programmer vs the garbage collector. And for most contemporary programs, correctness is the larger challenge then execution speed, especially with GCs which can run in parallel with your program logic, so can utilize unused CPUs "for free".
The latter might not bear a cost in CPU cycles, but certainly in programmer cycles, if the program logic allows for it at all. It also puts the burden of correctness on the programmer vs the garbage collector. And for most contemporary programs, correctness is the larger challenge then execution speed, especially with GCs which can run in parallel with your program logic, so can utilize unused CPUs "for free".