The mainmenu motion is based on the location of the mouse point relative to the stage height. There is not really a conventional "speed" setting to it.... There would only be some "delay" setting. Perhaps you have a link so I can see your menu?
You know you can set the menu to close_same_level? That will prevent it from getting too long and scrolling too fast. Find this setting in admin -> settings -> menu.mainmenu
There is not really any "speed", because it related to the relative value of the vertical mouse position. You can however improve performance a lot by reducing the height of your menu:
# Go to admin -> themes -> edit your theme -> menu.mainmenu.menuitem.vertical_textmargin ... Reduce this to a few pixels only, for example 2
# Additionally, you could decrease font-size, but I would try the above first as this may not be necessary.