make it using it as a passing lane difficult.
For example, you can't enter it when ready and then leave it as soon as you are finished passing. You have to wait for legal openings. That's to encourage its use as a cruising lane, not a passing lane.
Another example is the vehicles that are allowed to use it. Buses arguably have more right to use the carpool than anyone else. As commercial vehicles, they are almost always going to comply with the posted speed limit, even if traffic is flowing 10+ mph faster in the other lanes.
If you get frustrated about people driving the speed limit in the carpool lane the problem is 100% on you, not them. It's not a passing lane. It never has been. It's a cruising lane to encourage carpooling. The passing lane is to it's right. If you disagree then you are wrong and need to educate yourself. If you don't like it then you should convince your elected representatives to change the laws.