I looked at those numbers once and they are for different wind speeds. I'm pretty sure they are not in knots, but possibly in the Beaufort scale. Check that one out - you can get translations into knots in many places. Very few people sail dinghies in Bft 9, so that really means "5 and higher".
The idea is that based on design, sail carrying ability and ballast (hiking or trapeze) some boats do better in higher winds while others do better in lower winds. If your sailing venue doesn't have a good mix, then using one of the specialized numbers will be slightly more fair.
Having the right boat for the conditions helps. I was in a winning boat on a night where unusually heavy winds allowed us to profit from being too heavy in a slightly undercanvassed boat. The guys in the fast boat had to dump excess power, which made them slow.
What K.C. said.
Sailboat racing, to quote somebody I read recently, is different in that there's no "sprint' at the end. Most of the time, instead of making a big push to secure the finish, you focus early on, on not giving away your chance at victory, what K.C. calls minimizing mistakes. You try to be early at the start (not over early, but not in the 2nd rank), early at the first mark, early at the second mark and so on. You do this by optimizing boat and boat handling, but also optimizing the tactical and strategic decisions that are based on the state of the wind or the presence of "traffic".
After dabbling in "fun" races with the DS and another boat for a few seasons (as well as crewing at times on bigger boats in slightly more formal races), I find these statements a rather accurate summary. In addition, because wind-shifts can separate a fleet dramatically, sailboat racing really forces boats to stick together - gambling on a wildly different course rarely pays off (unless you have some very reliable expectation of where wind will appear, for example, because you happen to understand an unfolding weather pattern, while your competition does not).
Now, there are some "mistakes" that you can learn to avoid that don't require big expenditures.
The cheap thing to do is to put telltales on all your sails - so you can see the state of the trim. What
Arvel Gentry writes on the subject of telltales (and aerodynamics) is quite readable and illuminating. Or you can get a more basic "recipe" for sail trim and sailing to telltales in many places now. A wind indicator (windex) on top of your mast is most helpful in getting the basic relation of boat / sails / wind right, for fine control the telltales will let you know immediately whether your sails are working optimally.
On a DS hat has correct mast rake, you should have a balanced boat (not much weather helm, which means not fighting the boat's tendency to round up). Too much rudder is like a parking brake. Another parking brake is sitting so far back that the transom squats.
Fixing that might mean getting a tiller extension, if you don't have one already, otherwise it's free. I take it that you are hiking hard enough to keep the boat level.
Those are a few elementary things that affect straight line boat speed.
When it comes to tacking, there's a particular rate of turn that you'll find will preserve more of you boat speed. A really sharp turn will dump speed, a really shallow/gentle turn will have you use momentum for an extended period of going up wind. Experiment with an optimum - and read or learn about "roll tacking".
If you work on these, and your starts, you should be able to keep up with more of the other boats up to the first mark.
At some point, you would take critical stock of your hull and foils. If they are rough, scratched, banged up, they may need some TLC.
If you've practiced boat speed, you would learn to appreciate the difference even moderate improvements can make. Every winter season, you can do some project, like replacing a stock rudder with a better design:
or simply fair your CB and add some shape to it (several threads here).
The fastest boat on our lake doesn't always win. The local winds are tricky, and being in the right place at the right time isn't obvious. Sailors who figure out how to sail based on the wind and how to get around other boats better will often beat the "faster" boat. This effect is stronger in places with quirky winds and lots of traffic.
At some point, if you haven't already, you might want to make sure you have a decent set of sails. I think the sails that came with the boat may well have been several decades old. It was simply impossible to trim the jib w/o getting a flutter. If you are in this situation, spending a bit of money is advisable. If you race only occasionally and mainly for fun, sails will last longer than they will for serious racers, also longer if you don't race often in high winds.
I found that the trim for a decent set of sail was rather different than for my old sails, and had to unlearn some bad habits. So I would encourage you to make sure yours are at least adequate.