LTSpice multicore usage

Does anyone have experience in how effectively LTSpice leverages 
multicore processors? Is it also architecture/processor specific?

In the setup for a sim I can select how many cores I wish to devote to 
the simulation. I have a cheap Celeron netbook that I use when I'm out 
(the beater laptop) which has two cores (probably not the type of 
machine one should really be using, but it does okay on simple stuff
) but I often like to browse the web in the background and using both 
cores for the sim slows it to a crawl. Setting it to one seems to free 
things up.

I don't have like, actual numbers on the performance hit though so I'm 
just curious.