Your CPU is your weak point there, and running Windows isn't doing you any favors. The processor doesn't exceed 40% used because it can't. It's maxing out a couple of cores, and isn't properly multithreading. If you compare it to even a Xeon CPU from a few years prior, the Xeon will run circles around it. So will something equally old like an i7-3770. (and they're beating it with lower clock speeds and fewer cores at that!)
Other considerations would be Pagefile size, amount of space on the SSD, other programs using RAM, ect. More information would be needed to assist you further with this, but be aware that your CPU is a pretty huge bottleneck to overcome.