Its your cpu dude.. Its bottlenecken your system.. I should know.. I use to have an amd k6-2 533.. Well, use petes d3d pluggin or ogl pluggin and press the "FAST" button in the pluggin config.. Then change settings from that to get to your desired graphics.. Check for a balance between speed and video quality.. You can also try the software pluggins too.. And please read all the documents that come with the pluggins and epsxe.. Then check the epsxe FAQS.. This will help you a lot in configuring epsxe for optimal performance on your system..
I disagree, opengl is usually the best for a tnt card, unless you have the drivers improperly configured. The previous system I ran on was a p3-450 with a tnt2. Image quality also seemed better with opengl too, but those were the early days of the nvidia drivers so the d3d quality should be basically just as good.