Can you premake mojitos?
I don't know if having them pre-mixed would affect the flavor or not. One thing that could speed you up is having the limes already juiced. According to Dave Arnold at Cooking Issues people generally prefer lime juice after it has "aged" at least 4 hours. Maybe this changes with mojitos, it's hard to say.

This is assuming you are using fresh lime juice. Are you muddling fresh mint and sugar, or do you use simple syrup? You could perhaps make a mint syrup, then just throw a mint leaf on top as a garnish, that would save you some time.