The only way to reduce the load on your computer due to shared folder is by giving access to sepecific and required people. Well thsi can be a situation but sometimes you need to share some files publicly with many people. In Windows Vista there is a way to fix this.
When you share a folder select the advanced mode and you will see something which is pretty new. There is a number drop down which allows you to limit the number of simultaneous users on a folder. This solves the issue if you had been complaining about slow computer everytime you share something.