A great topic! :thumbup
IMHO, good speed is judged by the following criteria:
1. Having a reasonable relative speed to other traffic, which for me would be around 10-20mph either side of prevailing traffic speed, "prevailing" being those vehicles in your lane and the lane(s) either side of you. For instance, if the cars around you are doing about 60mph, you'd want to do somewhere between 45mph and 75mph.
2. Keeping a reasonable distance between you and the vehicle in front, a.k.a. the three second rule. The faster you go, the greater distance you need to have between you and the person in front of you in order to be able to stop in time if the person in front of you suddenly stops or loses control.
3. Having an "escape route" for the instances where you are closer than the three second rule. When you're preparing to overtake, for example, you may be closer to the vehicle in front, so you need to be able to change lanes or otherwise avoid the vehicle in front if they decided to suddenly stop or they lose control.
4. Observing the road conditions and understanding how those conditions affect how fast you can safely go. For example, if it's foggy and you can only see a couple of hundred yards ahead of you, be mindful of the fact that you may not be able to see hazards in enough time to be able to stop or avoid them. Same goes for high winds, wet roads, blind turns, darkness etc. The three second rule is useful in this situation, too... more than three seconds if you're dealing with rain, since that increases stopping distance.
5. Knowing the capabilities and limits of your vehicle. This is where I disagree with OP's original comment where it was said that a sportbike is not inherently safer than a Harley or a Vespa - a big hog can't corner as well as a sportbike, and a Vespa's braking and cornering abilities are dreadful! Not all bikes are created equal. For example, my husband used to have a Triumph Thruxton 900 which was kinda squirrely at high speeds, so he wouldn't take it much above 80mph. He now has a BMW R1100S and has been known to go quite a bit faster on that since it's solid as a rock at high speed.
6. Knowing the capabilities and limits of yourself as a rider. Don't have the ability to emergency brake within the distance you've left between you and the vehicle in front? Don't have the ability to swerve or stop at the speed you're going if you suddenly see a hazard in the road? Then slow down. Your safe speed may well increase as your skills increase.
These probably don't encompass everything to do with good vs. bad speed, but I feel like they're reasonable guidelines.