There is no rule, it depends on how good your shared hosting is. If you are on a server that is well managed, you will not feel any increase in speed, otherwise if your shared hosting is on a server with load issues, you will. People move to VPS for root access, not for speed. You should get a premium shared hosting - like the one offered by Liqudiweb etc. All under $10 shared hosts sucks, keep away from oversellers as well.