No, it's a fact, as it has been observed literally billions of times (subatomic particles in an accelerator decay at a slower rate because of their high speed). They even flew atomic clocks around the globe and demonstrated that they had experienced less time passing. This particular phenomenon is about as ironclad as you get in an experimental science...
Not to quibble too much, but the clocks on GPS satellites run faster than earth bound clocks due to the acceleration of gravity. Their actual speed has much less effect than gravity.