We got the first DC9 Super80's and had problems with captain climbing too slow or too high. It was so frustrating but as an FO you can only do so much. We were falling out of our 350 altitude one day and were told you have a PSA 727 at 330 5 miles in front of you opposite direction. I responded but we couldn't maintain altitude. We crossed over him at FL340 and they immediately cleared us down but heard the irritation in his voice. I told him you are too slow but he kept climbing.