Increasing the settings will obviously increase the work done by the program and increase CPU and RAM use. This is however, for the most part negligible, usually having the settings on High scans all files rather than those thought to be a greater risk, exe, dll, etc. rather than .txt, media files, etc.
Although you don't mention what resident scanners you are using, the one that might directly effect the internet is the Web Shield provider (the slider on Network Shield has no effect). For me even with web shield on High and using dial-up it didn't have any noticeable effect, some (but not all) on broadband have noticed a small decrease in speed.
But like everything in like it is about compromise, performance V protection and I know which side of the line I chose, protection.
With a system where system resources aren't usually a concern I doubt you will notice any difference moving to High for most of the resident scanners.
