If the logic is the same as the B777 (and I suspect that it is), FMC speed variations in cruise and within 10 knots, is programmed to recover the target speed at the rate of 1 knot per 12 seconds (5 knots per minute). Larger speed excursions are recovered to within +/- 10 knots much more promptly. The logic behind this is to avoid thrust settings constantly "hunting" for the sake of passenger comfort.
With the MCP Speed window open, the logic is to maintain speed fluctuations promptly (e.g. during approach), and thrust response to maintain the speed is almost instantaneous.
Hope that that helps.
Best Regards,
Old Smokey