Speed rating these days can be deceiving. Bows are advertised with speeds of 340 + F.P.S ratings. The thing you need to consider about these ratings is that they are achieved using I.B.O standards. Most bows, at least hunting bows, are going to perform closer to A.M.O standards.
I.B.O speeds are determined by shooting a bare bow with nothing on the string set at 70 pounds using a 350 grain arrow with a 30 inch draw. Most bow makers will test their bows with lighter arrows as well but most usually do not drop below a 330 grain arrow for testing purposes.
A.M.O speeds are determined by shooting a bare bow with nothing on the string set at 60 pounds using a 540 grain arrow with a 30 inch draw.
Speed is lost in a lot of ways. You can lose up to 5 F.P.S for everything you put on your string and 5-7 F.P.S if you use a whisker biscuit rest. So if you have a bow that is set up exactly to I.B.O standards that has a 315 rating and add a peep, a kisser button, silencers, release loop and a biscuit you are now down to 290. If you do not have a 30 inch draw you will also lose some speed. A shorter arrow even though lighter will not increase speed unless you are using an overdraw and still pulling the bow back 30 inches. Drop down to a more manageable draw weigh like 60 pounds and your down to 250 to 270 F.P.S depending on your bow and cam set up.
I own a bow with a 315 rating and set up with all these accessories except the release loop shooting 60 pounds with a 27 inch arrow that weighs 361 grains I am getting 240. I contacted the bow company about this difference and the informed me that the bow had only a 235 A.M.O rating so I should be pretty happy with 240.
When you decide to buy a new bow be sure you have the right information. Ask the right questions. Tell the dealer how you are going to set this bow up and ask what kind of speed your setup will achieve. If he doesn’t know call the bow company they will be glad to answer your questions. Bows are a costly investment so make sure you will still love your bow when you get it into the real world.