This is one of the things Roll20 specifically advises against. Characters should keep prepared spells on their sheet, and try to keep optional spells as few in number as possible. When you load a game in Roll20, all characters are loaded into memory, so that you can quickly reference them with macros, for example. Lots of spells times lots of sheets equals a great deal of extra data. This is the primary reason why when you buy a marketplace module, NPCs will have macros for their most likely to be used spells, but will only have a list for the others.