It’s not nearly unrealistic. Mirage 3’s acceleration is incredibly slow and bleeds energy like hell. It’s sustainable turn rate on low speed (below 400kts) is straight up disaster being delta wing.
The only thing keeping its br is the magic.
Edit: ppl just seems to have this impression that French delta wings always excel, well no, Mirage 3 is from the early 60s and has a rather pathetic engine plus absolutely nothing optimized for lower speed, it can’t dogfight most jets it meets after the first window of attack.
So let’s say if you would ever see something like a F8E that got flare, all you get is one chance firing magic 1, and that is it.
When it’s slower than 300kts, it’s pretty much a brick like F104, which I’m not even exaggerating.