Fast growth isn’t always a red flag, but it depends a lot on how that growth is happening.
If it’s coming from things like strong content, good internal linking, or hitting the right search intent at the right time, then it can be completely natural. Sometimes a single piece of content can gain traction quickly and pull the rest of the site up with it.
On the other hand, if the growth is very sudden without clear reasons (like no real content depth or weak user signals), it can be less stable and more likely to drop later.
I think consistency is the key indicator—if rankings hold or grow steadily after the initial spike, it’s usually a good sign the growth is more sustainable.