There are a few things that you can do to optimize the speed at which symbols draw in your maps.
Consider simplifying symbols. Complex symbols or multilayered symbols draw slower than simply constructed ones. Symbols that contain PNG (*.png), JPEG (*.jpg,*.jpeg), or Windows bitmap (*.bmp) picture files and halos usually draw slower.
Consider using symbols from the ESRI_Optimized style. It contains both line and fill symbols that are designed to perform better while still maintaining a close visual appearance to their counterparts in the ESRI style. The line and fill symbols in the ESRI_Optimized style draw almost 60 percent faster than those in the ESRI style.
The drawing performance of marker symbols, whether used to symbolize points or as a component of one of the other types of symbols, can be improved greatly by using simple marker symbols and single-layer character marker symbols; these are the best choices for symbolizing large point-feature datasets.
Masks can greatly increase draw times. A good alternative to adding a mask to a character or simple marker is to create a .emf file that represents the symbol and uses a picture marker symbol that displays the contents of the .emf file. In addition, .bmp, .png, .gif, and .jpg/.jpeg-based picture marker symbols can be replaced with .emf markers to improve performance with those symbols.
The more complexity in line symbols, the more potential for compromised drawing and export performance. Of course, this needs to be balanced with the need for the degree of detail necessary for proper cartographic depiction and communication. Simple line symbols are the fastest to draw but offer limited display options. Cartographic line symbols may draw slowly if they are constructed with a lot of complexity. Multiple layers, pattern templates, offsets, and excessive line widths will all increase draw time, for example.
In addition, combining symbol types into a multilayer symbol can slow down performance. This is because the rendering of the dataset must be cycled through once for each layer type. For example, if you combine a cartographic and hash line symbol, as is done in a basic railroad-style line symbol, the data is drawn once for the cartographic symbol and again for the hash symbol.
Picture line symbols can dramatically increase draw and export time and generally do not produce aesthetically pleasing results. It is recommended that you use one of the other line symbol types instead.
To maximize drawing speed, use simple line symbols for polygon outlines wherever possible. Only use cartographic line symbols for fill outlines if you need the enhanced properties they provide. For picture fill symbols, .emf pictures will draw faster than .png, .jpg/.jpeg, and .bmp pictures.