If it's exposed as a uniform, it should be possible to adjust the blur intensity automatically depending on the time it took to render the last frame. I'm of the opinion that effects should be as FPS-independent as possible 🙂

For some games, it makes sense to use a stronger motion blur when capping to 30 FPS. That said, this is better done with a setting exposed to the player (which would become a multiplier of this automatic FPS-based intensity).

What I specifically did, and I'm not saying this is correct, is that I scaled the angular velocity of the camera based on the framerate. At least in my case that made the blur be fairly consistent based on a reference framerate of 140:

var quat:Quaternion = Quaternion(_camera.global_transform.basis)
var diff:Quaternion = quat - _last_camera_rotation
_last_camera_rotation = quat
var velocity:Quaternion = (diff * 2.0) * Quaternion(-quat.x, -quat.y, -quat.z, quat.w);

var velocity_scale:float = min(1 / delta / 140, 1)

_postprocessor_material.set_shader_parameter('camera_angular_velocity', Vector3(
    velocity.x * velocity_scale, 
    velocity.y * velocity_scale, 
    velocity.z * velocity_scale))