minimize start up items

I was told that if there are many start-up items(e.g. 10 items appears on the manual bar) which are not used and let it stand to run, it slowes the computer operation and takes up virtual memory which is not worth it. Is this true? If so, please advise how to minimize this or how to do the procedures in order to make sure they don’t get run if they are not used during operation. many thanks for your help.