The factor that is NOT relevant to improving an algorithm's efficiency is "number of lines of code." While the number of lines might give a rough indication of code complexity or readability, it does not directly correlate with the efficiency of an algorithm in terms of time and space performance. Efficiency is typically evaluated based on time complexity (how the runtime increases with input size) and space complexity (how memory usage increases with input size), rather than simply counting lines.
Therefore, the correct answer is number of lines of code.