I have owned a Fascinate since the day they were released.
Yes, it will get very slow if you try to run apps w/o turning off the previous app. ie, clearing the RAM.
install Battery Doctor, create the Widget it provides and put that on your home screen. In Settings/Whitelist make sure that only the bare minimum apps are allowed to run after cleaning. This will help you a lot, and the battery will last a lot longer too.