The galvanometer is driven by current flow which will generate torque to drive the rotational motion of the needle.
The ineritial of the needle will determined the response time.
If the ineritial is too large, the needle will move slowly.
if the movement of the manget change direction before needle reach it's maximum amplitude,
the current flow in opposition direction, then the torque will change direction.
So the amplitude will become smaller than it's possible maximum value.
You are welcomed to check out my [url=http://www.phy.ntnu.edu.tw/demolab/phpBB/]Physics discuss forum(in Chinese)[/url] if you can understand Chinese.