It does seem excessive.... Mine never leeks and I often follow my fiance around in the same manner as you. A couple of things to consider.
1) Your radiator cap is defective, try one off of another bike for a few laps to check, if it is get a new one. Does it boil over when you are riding at your pace? If not rule this one out.
2) Head gasket is leaking pressure into the cooling system, does it boil over when you are riding at your pace? If not rule this one out.
3) Use a good synthetic oil in the motor, I use Mobil 1 15W50
4) Run your coolant mixture with less antifreeze, more distilled water. It is more efficient for cooling 30/70 or even 20/80 mixture.
5) Go to a Kawisaki shop and order a 1.8 cap for the KX500. The KX5 standard is a 1.5 I think but they make a 1.8 too, I had to go to this on my punched out YZ400.
6) are your radiators in good shap? The CRF rads are easy to bend and if the fins don't get sufficient airflow they won't cool properly. If they are in good shape rule this one out.
Hope it helps.