Scan all downloads will only run when a file is downloaded and before it is moved to the destination directory. Depending on usage, most people will not see a whole lot of slowdown occuring from this unless they are downloading large files all the time.
Monitoring file and program activity. I'm not sure what the options are here, but this is most likely to be the most effectice option to adjust. If there is an option to only check certain types of files or files that have changed, they may help speed things up a bit.
Behavior monitoring checks each and every activity against not just a set of virus definitions, but also a set of definitions of how viruses look. It's called heuristic analysis in other packages. An analogy would be looking not just for people who are terrorists in airports, but looking for people who look like or act like terrorists (I know, horrifically politically incorrect analogy, it was just the first that came to mind).
Network inspection probably ends up inspecting each and every data stream in and out of your computer. If this is a business network that has network security appliances already in place, this can be turned off. If it's a home network, or an unmanaged network, it should stay on.
UPDATE response to comment:
I would not modify which files it scans. Viruses are capable of placing themselves in any number of locations. It is best to leave the scanner running on all files.
Regarding while file to monitor, I'd select Monitor incoming files only. So long as you know the system is clean at this point, this setting should protect against incoming infections. When a system is infected though, you'll want to remove it from the network as soon as possible to prevent the infection from being sent out to other systems.
1This is going to depend on exactly the machine and the usage, so it's hard to provide a general answer. If you're not downloading files, scanning those is meaningless to performance, if there's no network activity, scanning the network is meaningless to performance, etc. – Shinrai – 2011-08-10T16:57:44.210
@Shinrai: Almost always it is a Windows XP Home machine used at home, visiting facebook and basic websites and doing instant messaging and basic word processing. – William C – 2011-08-10T17:21:15.340