How much the performance will decrease depends on the number of snapshots in the used chain. Assuming you create all the projects (snapshots) from the base disks you will only have two - maybe three - virtual disks in each single chain, and depending on your system you may only notice a difference in performance using benchmark tools.
To switch between the projects you need to understand how snapshots work in VMware products. Before you "Go To" another position in the Snapshot Manager, you need to save the current work "You are here" by creating a new snapshot. In order to not end up with too many snapshots in a chain, you'll have to "Delete" the additional snapshots to merge the delta data to its parent (project) snapshot.
André