Doubt about Raycast Performance

Hey everyone. I'm helping in the programming of an A.R game for the iOS and I'm using a bit of raycast to get random positions off of the camera limits to spawn the mobs with camera.ViewportPointToRay.

I'm generating raycasts for bottom-left and right and upper-left and right corners to get the collision point of the ray and then going through the process of randomly choose a position to instantiate the enemy. This process is done each 4-7 seconds.

Did you guys think that this can cause some performance issues when running in an iPhone? There's other way I can get an off limits of view position without using raycast if this way is too expensive?

The point in use a constant limit is that the player will be constantly moving the phone and that would cause some strange instantiation.