I think you are referring to these numbers:
Squeak 4.2.4beta1U, MVC image, OS X 554844390 bytecodes/sec; 12213718 sends/sec
RoarVM, MVC image, OS X, 1 core 66286897 bytecodes/sec; 2910474 sends/sec
RoarVM, MVC image, OS X, 8 cores 470588235 bytecodes/sec; 19825677 sends/sec
I'm holding off on drawing any conclusions from these numbers because I'm not sure where they come from and what they were run on. I'm not even sure they were all run on the same machine. Taken alone the last two look okay as adding seven (7) cores adds a reasonable amount of processing speed. Looking at the first two, I find it hard to believe there is such a large drop in processing speed. If it is true, something is very, very wrong.