Hi.
How did you find out the GPU usage %? Did you use a tool like CPU-Z?
In my i7 @2.2Gz laptop a main loop is about 5-13%(one core).
One suggestion is after loop processing to write:
sf::sleep(sf::milliseconds(1));
This enables the CPU "rest" a bit between the main loop cycles and return control to the OS.
In my past games/apps i was around 20-30% and after i used this it dropped to the aforementioned %.
Edit: One more thing i discovered some time ago is that i had downloaded "modded" GPU drivers(my gfx card was itegrated graphics) and although CPU was around 20% i was rendering like 2-3fps. When i removed the modded and installed the official ones it worked normal.