I wonder if it might be a good general rule to never give just 1 CPU to any multi-threaded application (JVM or otherwise).
Often, there's a mix of threads, some of which are doing CPU-intensive stuff and some which just need to do some quick thing to unblock something else, like start some new IO when some IO completes. With 1 CPU, any time any CPU-intensive stuff is happening, these quick things have to wait their turn until the next time slice. With 2 CPUs, you have twice the work and twice the likelihood of having to deal with CPU-intensive stuff, but CPU-intensive moments don't necessarily happen at the same time, so you have better odds of having one of those CPUs immediately available to do the small, quick stuff.
Often, there's a mix of threads, some of which are doing CPU-intensive stuff and some which just need to do some quick thing to unblock something else, like start some new IO when some IO completes. With 1 CPU, any time any CPU-intensive stuff is happening, these quick things have to wait their turn until the next time slice. With 2 CPUs, you have twice the work and twice the likelihood of having to deal with CPU-intensive stuff, but CPU-intensive moments don't necessarily happen at the same time, so you have better odds of having one of those CPUs immediately available to do the small, quick stuff.