Many hands, one job
A GPU helps most when several pieces of work can happen without waiting for one another. That is parallel processing.
Which job lets everyone start
Drawing A ∥ Drawing B ∥ Drawing C
Clue 1 → Clue 2 → Clue 3
Independent work versus waiting
Coloring 100 separate drawings is easy to share. Each person can start immediately.
A treasure hunt is different. You must solve one clue before knowing where to find the next. More helpers do not remove that dependency.
Computer work has both patterns. The useful question is how much of this particular job can run together.
Handing out work also takes time
The computer must prepare the task and make its data available. For a tiny job, that overhead can outweigh the calculation time saved.
So “a GPU has more processing units” does not automatically mean “this program finishes sooner.”
Try it yourself
Which task is easier to divide: brightening a million independent pixels, or following a chain where each answer determines the next input?
Check your answer
The pixels. The chain has dependent steps. Some parts of an application may suit the GPU while other parts remain on the CPU.