Road runners have it easy - they can compare their performance from race to race and training session to training session just by running the same distance.
Orienteers have a problem. We never run the same course twice. Our race distances are approximate at best, and measured along a straight line that hardly any of us follow.
So how do we tell if we are improving? How do we compare our performances from one event to another? We use the ratio of minutes per kilometer. If our course distance (always measured straight line) is 5K, and we take 100 minutes, then our min/km speed is 100/5, or 20 min/km.
What's a "good" speed?
At the "classic" (regular) distances, the best in the world do 4 or 5 min/km, the best in the US do 7 min/km, the best in GA do 10 min/km. A good speed at a local GAOC meet (not a race) would be 15 min/km. An average speed at a local GAOC meet is close to 20 min/km.
Another variable that affects our performance is the climb of the course. Climb is the number of UP contours along the optimum route (as determined by the course setter) multiplied by the contour interval. We divide the course distance in meters into the climb in meters to get the percent climb. A hilly road race course like the Peachtree Road Race is about 0.5% climb. Rarely do we have that small an amount of climb at an orienteering event. We typically have 2% to 4% climb at GAOC events, with 4% being the USOF suggested maximum. For instance, a 5K course with 200 meters of climb would have 200/5000 = 4% climb.
Some people, unsatisfied with their min/km performance, factor the climb into the equation to give them better numbers. In that case, multiply the climb by 10 and add it to the straight line distance in km before calculating your min/km.
For instance, a 5K course with 120m climb that you finish in 86 minutes would be: 86 / (5.0 + 1.2) = about 13.9 min/km. Without the adjustment for climb it would be 86 / 5 = 17.2 min/km. Personally, I would not bother with the adjustment for climb. If you adjust for climb, then why not for thickness of terrain or difficulty of navigation? It is too inaccurate to be used except to generally say that a particular course was steep or green and that affected your performance.