Theoretically, yes, a little. However, people typically report that responsiveness is not affected by measures like these. I have even heard of people pouring molten lead into their bars, filling them to the brim, (a few times the mass of our bar-end weights) and saying their handling was unchanged. Why wouldn't you notice a slow-down in turn initiation? A way to understand this is with the concept of resonance, and its effect on the ability of the bar ends to change speed, discussed above. The fundamental frequency of the engine (67 Hz, for a single, operating at 4000 RPM) is at least 130x the frequency of a rider's steering input. (0.25 - 0.5 Hz.) That means that a weight which is sufficient to seriously discourage the engine resonating with or flapping the bars, is inconsequential to the much lower frequency involved in steering the motorcycle.