Check that wiki, I bet there aren’t many PS2 games that are playable without speedhacks. If you can run Tekken 5 at 50FPS, maybe try the PAL version and see if it’s smooth.
Running the PAL version at 100% speed compared to like 83% speed would be better. Although I’m not really sure if it scales like that…
At native res it shouldn’t be so taxing. Do as @pzlate mentioned and check the database for best settings. Something is probably wrong in your config because I recall PCSX running pretty good and I have a much weaker PC.
I remember those Namco games being pretty buggy, but yeah, I was able to play games at full speed with some speedhacks enabled with an E8500 dual core. This was all 4+ years ago though.
Also that build you’re running is like 3 years old, I wonder if you could take the new plugins from the latest build and use them in the old version to see if if things run better.