The point is, if you were to take Minecraft and recode it, line by line, in C++, without making any changes, it would still run like crap. Sure the compiler might be able to optimize a bit here or there, sure the runtime would run faster, but neither can change poor programming or concept/design decisions. On the other hand Optifine, which fixes some of the worst offenders, can increase FPS several times.
Yes, well-written and well-optimized code would probably run very slightly faster in C++, although not by that much. But Minecraft (or, frankly, Forge and many mods) is neither of these.