For the EC series, hold a side button with the hz you want to use and plug it into your PC. Don't let go of the side button until the wheel lights up. Use software to confirm that your polling rate has changed to the speed that you selected.
Plugging in the mouse with no buttons held = default 1000hz
Plugging in the mouse while holding the "page forward" thumb button = 500hz
Plugging in the mouse while holding the "page back" thumb button = 125hz
I would suggest trying out 500hz, it makes a huge difference over 125hz.
EDIT: It seems that you meant how it tracked at 125hz..... my bad.