I assume you're referring to VSync?
If the system is able to produce significantly higher FPS that your monitor's refresh rate, then yes, enabling VSync will result in less load on the GPU, which in turn asks less of the CPU.
A standard VSync with double buffer has - as the name suggests - two "buffers" which store completed frames ready to be sent to the monitor. With VSync on, once a monitor starts drawing the next frame, the GPU will immediately prepare the following frame as quickly as it can. But when that frame is ready, the GPU has both buffers full, the Front Buffer is displaying the current frame, and the Back Buffer has the next frame queued and ready to go. In that case the GPU will stop working until monitor completes drawing the current frame and moves to the next. At that point the Back Buffer gets moved to the Front and displayed, and the GPU fires up again and begins preparing the next frame.
But yes, if the GPU can prepare the following frame much quicker than the monitor can actually draw it and ask for the next, it will sit idle for that period of time, reducing load on both GPU (significantly) and CPU (to a lesser extent).
The CPU, of course, is still doing Audio processing, AI, etc, etc, none of which is FPS dependent, so VSync has no impact there. But the CPU is still working on supporting the graphics cards so lower fps (which is effectively what happens with VSync on a system which could push out many more frames) will lower CPU load to an extent.
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