The trackball will move your cursor much slower than your normal mouse would, that is normal. You don't need to adjust the speed in your operating system if you just want to change its speed in MAME™ as MAME™ takes full control of the trackball when it's running.
Adjusting Speed in MAME™:
MAME™ has its own analog sensitivity settings, and usually the default setting is fine. But to adjust this setting (while playing any MAME™ game) press the TAB key on your keyboard and go to 'Analog Controls'. Write down the default settings there and make adjustments to your liking. If you want to revert, just use the settings you wrote down.
Adjusting Speed in Windows™:
For Windows 10™ just click on the Start button and type 'mouse' to find 'Mouse Settings', then click 'Additional Mouse Options'. Otherwise go to the "Mouse" section of your Control Panel in Windows™, under the "Pointer Options" tab adjust the "Select Pointer Speed" bar to faster for more sensitive movement, less for slower movement.
This menu may be slightly different if you have 3rd party mouse software installed. Remember that this adjustment will also affect any other mouse you may have connected.
More information on mouse polling here. (Mirror)