Jack: You don't get the same framerate. You will get close to same framerate if one type of task is very slow but it will not be the same. If five task are done in parallel, then the slowest task will decide total speed. But if those five tasks are done in a series the total time will be the sum of all those five tasks.

Yes! This is very easy. But you don’t seem to understand? If you think it is like the weakest link in a chain isn’t right.
The sample you gave about hous, lumber and nails. That sample is a parallel situation. This situation isn’t like the gpu.
It would be more right to say that the developer creates the lumber and while he does that he can’t use the nails to build the house from lumber and nails. When the lumber is ok then he can build the house.

Lumber for one house = one week
Nails for one house = one day
Total time for building house = one week + one day for the developer