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.