Legally, you are required to stop because there is always a stop sign at the checkpoint. Following that, you legally are prohibited from exceeding the speed limit as you drive away (
18 USC 758). I'm genuinely curious what would happen if significant numbers of people started to approach checkpoints, slow to a complete stop, ignore the agent at primary inspection, and then proceed safely forward at just under the posted limit. I'm not personally aware of any law that would be violated in that instance. I know 18 USC 111 has been used successfully in the past under the 5th Circuit, but that charge definitely wouldn't stick in the 9th Circuit.
And just FYI, I inquired about this exact scenario when I was going through the academy. While studying the "high speed flight" law in my Applied Authorities class, I asked my instructor, who was a CBP lawyer, what if someone just drives away from you while you're working the checkpoint, but they don't exceed the speed limit. He shrugged his shoulders and said, "Good question." This from a lawyer employed by the CBP Office of Chief Counsel.