Mapping Center Answer:

Good question! We actually have a couple of presentations on our Other Resources page that you can download to get more info but here are the short answers to your questions: 

The ESRI_Optimized style contains both line and fill symbols that are designed to perform better (for display, exporting, and ArcGIS Server) while still maintaining a close visual appearance to their counterparts in the default ESRI style. (The symbols in the ESRI_Optimized style show an average improvement of about 60 percent in draw times over those in the default style.) They use simple line symbols as the outline and the picture fill symbols use .emfs versus bitmaps. So if you modify your symbol and do not use a simple symbol then you will in fact affect the performance of the symbol.

In terms of your point about separation, for example -- if you make the separation smaller, it will draw more slowly (because more lines have to be drawn); a larger separation will improve the performance.

And the colors are not web safe, which for most monitors is not really a major consideration anymore.

If you want to learn more, there is a longish (75 minute) presentation called "Getting to Know Symbols and Styles in ArcGIS" that we gave at the 2007 ESRI User Conference -- this one has full bottom notes. We gave a shorter (20 minute) presentation at the 2007 NACIS conference during Practical Cartography Day (PCD) called "Symbols and Styles in ArcGIS" -- this one doesn't have the bottom notes.