I have Maintain Speed checked, but the camera still appears to speed up at times. Why is that?

There are two illusions of speed that can make Maintain Speed appear to speed up or slow down the camera.

Camera rotation—Maintain Speed doesn’t account for camera rotation. As your camera is moving through space, the rotation of the camera can create the illusion of speed as well. To minimize this effect while using Maintain Speed, reduce the number of changes in camera rotation along the animation path. Commercial Product Video
Closeness to the ground—As you move over the surface of the earth, the camera will appear to move faster when it flies closer to ground. You can consider using a consistent height above the ground to reduce the effect, or slow down the overall duration of the animation.