The original Minecraft versions had reasonable performance for a one-man app that was more about experimentation, because Notch wrote his code in a way that did

The original Minecraft versions had reasonable performance for a one-man app that was more about experimentation, because Notch wrote his code in a way that didn't put too much pressure on the GC. For instance block coordinates were passed as individual x,y,z arguments to each function. When Notch passed on maintainership of the code, he apparently didn't pass on his knowledge of how far you can push the JVM, and the new developers proceeded to "modernise" the code, by, for example, introducing a Point class. The amount of garbage Minecraft made went up very quickly as a result and it got slower. There were a bunch of other glaring problems ... the fact that Optifine is able to improve it so much shows, I think, that the developers didn't fully understand Java performance.