Performance of combo mouse on serial port

[TheManuel]

I'm going to give this issue a second try.
I own a CompUSA Crystal trackball which works on both, the serial and the PS/2 ports (it has a PS/2 to serial adapter).
However, its performance on the serial port is inferior.
Eyeballing it, it seems to report mouse location half as often as when connected to the PS/2 port.
I bought another combo mouse and it suffers the exact same problem.

Does anyone know the reason for this?

Would a dedicated serial mouse work better or do they simply suck?

Thanks.