The mapping speed slider modifies the Network Timeouts, set in the Network Tab of the program options dialog box.
Slower devices and devices separated by several hops require more time to discover than faster, closer devices. Setting the slider to "slower" gives greater accuracy.
Mapping switch and hub ports also takes time. You can significantly speed mapping on large networks by if you uncheck the Map switch/hub ports option on the Miscellaneous tab of LANsurveyor's application preferences.