I fly 121 at a large regional. As others have said it is very standardized and dispatch pretty much does most of the planning of the flight (routes, altitudes, fuel requirements, etc.) and, as a crew, we merely look it over. We arrive at the gate, receive the paperwork, which includes the flight release and weather/NOTAM package, and then review it to ensure that we agree that the flight can be completed safely as planned. Many times we have new dispatchers (not a bad thing, just the nature of the beast) who may miss things or have a difference of opinion with the PIC, so it is not uncommon to rework some stuff with the dispatcher to come to an agreeable situation. Almost always this consists of a different alternate airport, getting more contingency fuel, or changing a route due to weather or turbulence.
In our paperwork package, we also receive what we call an "MGL" or "Max Gross Load", which is essentially a document that tells us how much weight we can accept for various runways based on environmental conditions. This is normally just "info only" for planning the flight, or helping us decide which runway(s) we can accept while we are preparing the aircraft. Once it comes time to actually do the W/B calculations and takeoff performance, we use ACARS. We input the passenger numbers and cargo information, and it then sends us a report with our W/B info. It gives us our weight, our CG as %MAC, and trim settings for T/O. We then send off a request for a specific runway performance. This report basically tells us the maximum weight in order to use the runway based on balanced field, obstacles, etc. It also provides our v-speeeds. So, under normal ops, performance for us is done in a matter of seconds just with the computer.