Java is interpreted language, which is not good for big applications, especially games... And it consumes a lot of memory. To give you example, Mythruna with 128M clipping, and default shading quality (high) runs on my computer with max 20 fps when looking straight forward, it is worse result than CoD:MW3 (on lowest details)... After changing shading quality and reducing clipping distance, it runs good though. But it takes more memory and CPU than World of Warcraft...
First, Java is not an interpreted language. You are working from 15 year old myths. Java compiles to native code on the fly for the platform that it is being run on and then recompiles the often run sections (hotspot) to improve performance. Theoretically, this means that Java can run faster on some platforms than C++ if the C++ app was not specifically compiled for that platform. In fact, back when CPU usage really mattered, games tried to get around this issue by compiling their games multiple ways and dynamically loading the best version for the platform.
Second, the speed is almost entirely down to the power of your graphics card. I get over 200 FPS on most of the world because I have a nice card. (Note: on the public server around spawn town things are really overbuilt so even I only get 40 FPS these days until I walk a ways off.)
Third, the nature of a block world game means that it is going to take more memory than those other games that are optimized to specific geometry. The game has to keep the block representation of the nearby world as a giant 3D array of data and then it also has to keep around the "mesh" that is rendered. I believe that Mythruna's memory requirements are similar to that of Minecraft's at this point. At least more similar than the other games you mentioned. To be honest, I haven't run Minecraft in a long time but it is a better example of a comparable game. You can actually run Mythruna limited to 256 meg (by running it with a special command line option) but you have to limit the clip to 64 meg or you will get frequent pauses.
Mythruna is doing more work than CoD or WoW because at any time you can start removing the world. And in fact when you are standing still looking at the horizon the CPU isn't doing much work at all. Look up at the sky and watch your framerate shoot up. This is because all of the real work is being done by the graphics card and now there isn't as much for it to do.
If you press F11 you can turn on full stats and see the number of triangles I'm pumping to the graphics card. The count is quite high when looking around... until you look directly up or directly down. Then those counts should go way down and your frame rate should shoot way up.
Mythruna's graphics engine really isn't that optimized yet. I will improve things for slower platforms once I reach alpha. I've talked about why in other posts.