What fps can be considered as a hardline for real-time performance? Is there any academic paper that describes this?
There are as many ad hoc "definitions" of "real-time" are there are people or places you hear or read about what it supposedly means. Some people claim there is a "hardline" such as 10uS response time, but I believe there is no academic paper that makes that mistake. 

Many people implicitly have an informal mental model that considers a system as being "real-time"
•   if, or to the extent that,
•   its actions are manifest to action observers with latencies that can be related (usually in an application-specific way) to the actions’ perceived currencies—
•   i.e., in a time frame that those actions’ latencies and predictability of latencies have acceptably satisfactory value to their observers (e.g., system users). Think of high frequency trading.

The magnitudes of the time frames are application- and situation-specific—e.g., microseconds to megaseconds—not related to if, or to the extent, that a system is a real-time one.

So a system "operates in real-time" if its actions have low enough latency to satisfy your needs. If 15 FPS is a satisfactory enough rate, then the system is real-time as far as your needs are concerned; otherwise it is not--that's all that matters to you.