I have the same problem exactly.
Jupiter-8V uses 50% of the resources of the CPU, when I only use it in Cubase SL3.
It is a pity because it has very good sounds.
However I cannot use it with other instruments, because it uses too much
of resources CPU.
I hope that Arturia will optimize its software.
I use version 1.0.3.
Processor: Intel Pentium 4 (1,7 GHz)
RAM: 1,5 MB
Sound Card: EMU 1616m