What is the most time efficient way to serve food fairly to ~100 people?
Seems like a good use for "family style". Bring platters or bowls of each dish to each table. Then at that table expect decent behaviour such as passing the platters around so everyone gets some. It shouldn't take long to fill and distribute bowls or plates of things. Carving meat into slices might take a while but would have to be done even if people were lining up to get their individual portions.

If some people weren't good sharers you could use assigned seating and put the non sharers at a table that gets its platters last.