We try to get in fixes for performance in the shooting range, but generally we keep it at a lower priority than the main game mode.
This issue is probably related to a sort of consequence of the game's architecture. We need to do something called "garbage collection" periodically, which is where we tell the game to kick out stuff that's loaded into memory that doesn't need to be there anymore. In normal modes, we force this to be done on the round resets so that it doesn't impact gameplay at all (which is also why you might see your graphs dipping when the round swaps over to a new one). This is why you might get this issue in the range but not in actual games.
In the Shooting range, there's never a round reset, you're just chillin' there. Eventually you'll get to a place where memory is capped out, so when something needs memory, the game needs to dynamically kick something out and replace it with the new thing, which can cause dips like these. They aren't usually this bad, so I'm not really sure why it's like this for you. We will eventually run garbage collection while you're in the range, but I believe it's on a timer or something, I'm not 100% certain what triggers it.
If this is happening to you, a simple workaround is to leave the range and re-enter it (like go back to the main menu). This should force garbage collection to happen and let you re-enter the range for a while before this starts happening again.