When there are lots of spells in character sheets spell list we seem to get quit a bit of lag.

As wizards, clerics, and druids advance in level using the 5E character sheets, my players add all the spells their level can cast or that they have in their spell book and then mark those spells they want to have prepared after each long rest.  As they reach higher levels there are a lot of spells in their list of potential spells.  We have noticed there is quite a bit of lag when they want to open their character sheets when there spell lists get large.  Any ideas?

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.

Thanks, I talked to the wizards of my groups and they say they understand.  All of them have decided they will put up with the lag in order to keep their full spell books available.  Helps to know why so they can make an informed decision.

If that tradeoff works for your group, you can try optimizing elsewhere in the game to make up for the hit. The  Optimizing Roll20's Performance  page has some good advice. And in many cases, the  Best Practices for Files on Roll20  page can help, too.