Apparently many people have had this problem specifically with the Mac version of Fallout 2 from GOG. It has mostly to do with the fact that it uses Wine, which is a Windows based program to run old games (similar to DOSBox which is what Fallout 1 uses for Mac) but there is a compatibility interface issue with the OS X mouse support with Wine so that it moves super sluggishly slow in FO2. As for fixing it goes there are some suggestions I've found in other topics on the forum, which seems to have something to do with renaming a .dll file (also known as a wrapper I guess) in one of the directories for the game... I don't really know specifically what to do though.