What you're (99% sure) referring to is the
acceleration curve of the mouse. Mice in most operating systems are non-linear. That is, if you move your finger twice as fast, you get more than a 2x increase in pointer speed. This lets you use the same trackpad or mouse to move all the way across the screen without running out of space and still make accurate fine grain changes.
The acceleration is different from the
speed or velocity. That's why adjusting the mouse speed doesn't satisfy you.
There is an element of personal preference to acceleration curves, but they are also somewhat different in every major operating system, and many people become really accustomed to whatever was in their original operating system. So I for instance got really used to how the Amiga handled its mouse, because it was the first time I used a mouse intensively.
OS X does
not have much in the way of built-in tools for this, but since you're talking about the external mouse, you might be able to benefit from a different mouse driver that lets you adjust acceleration.
Steermouse is one such popular option.