I have a '93 honda accord lx. At speed there is very little heat coming...
 

...out of the vents. The temperature.. gauge stays in the cold section until you are stopped. Then it starts to climb, never overheating. Antifreeze is full. Could the thermostat be stuck open? That's my guess, all answers welcome.

Yes, your thermostat is stuck open ... it likely is just a little open but the high pressure means it might as well be all the way open ... have it replaced even if they say it is okay ... if by chance it doesn't fix the problem it is still a good thing to have done ... but, I have had the same problem about three times in the last 10 years and it was the same answer each time.