Why is this considered a "bug"? It might be a little slower in some cases, but of the behavior is correct for a set of inputs and outputs, the classification is a stretch.
The designation of what should be counted as a bug is less clear for performance bugs but if some operation is slower than it could/should be, and if this slowdown is significant enough to matter, and if fixing the slowdown is quite practical then, yeah, I would definitely call it a bug. I have filed many bug reports for performance issues over the years, and many of them have been addressed.