GetFrameTime() is a *clock* but the result will be different depending on computers speed
Which is exactly what you want, isn't it? The faster a computer renders a frame, the fewer movement happens -- and vice-versa.
You can use GetFrameTime() as a factor. For example, if the target FPS is 60, you can get a factor by calculating GetFrameTime() * 60, so that you get a factor of 1 when rendering with 60 FPS. When the FPS drops below 60, the factor will be greater than 1, so movements will be faster.