I assume you have a data sheet for your LCD. First, only update the screen when a value changes. This means tracking "OldVolts" through a separate variable. Next, when you DO update, only update the new values; in other words, don't update the whole screen, don't update the whole line, in fact, only update the actual characters that have changed. Another common issue is sending the Erase Screen command every time you want to change something. DON'T! Just update the changes. (Again, it is explained in common LCD data sheets how to change only 1 DDRAM address without having to reload the entire screen.)