Here is my setup,
I run literumble clients on two machines almost non stop. Each runs one rumble and one melee client at the same time. Since one has 4 cores and the other 6, this should not be a problem with CPU calculations under normal circumstances. Also, there is plenty of RAM so no swapping. If I expect high CPU demanding tasks, I stop clients so they not affected by CPU fight with other programs.
All my machines are 64 bit linux with openjdk-8-jdk-headless.
But I think at some point robocode became multi-threaded, so CPU constant (6.5 ms in my case) is not so relevant anymore. In top, I can see that rumble client often eats more than 100%, i.e. it spans between cores. Also, load on a machine seems to be high, i.e. it is often in the range of 3 for the 6 cores machine.
Plus there is CPU throttling which kicks in when ever it wants, thanks to modern hardware design.
I don't think Robocode actually runs more than 1 thread at a time. I think the extra CPU is from the JVM JIT optimizer running in parallel.