A lot of optimizations are off by default, or completely unavailable because he's using JDK 1.6. See https://github.com/jruby/jruby/wiki/PerformanceTuning

Startup time is slow because it needs to warm up the JIT. Same with Rubinius, which is mysteriously missing from most of these graphs.

Memory consumption partially depends on how much you tell the JVM to use - it'll run the garbage collector more or less aggressively to keep within that bound - default appears to be min(system memory/4, 1GB).

That's not to say it doesn't need more memory than MRI, but you should factor in the concurrent native threads allowing one process to serve many clients, and the precise compacting GC limiting memory fragmentation and associated heap bloat with long-running processes.