Here are the side-by-side WEI scores for all systems. The top group shows scores for my Mac Mini; the bottom chart shows the two MacBook Airs.
All of these scores are on a scale of 1-7.9. The color coding is simple, bright green is the best, dark red is the worst, with yellow in the middle. The two MacBook Airs have different CPUs, but both have the same 128 GB SSD and Intel onboard graphics. The default VM configuration sets aside a mere 1 GB of RAM for the VM. For the optimized setup, I increased RAM to 3 or 4 GB.
You can see at a glance that virtualization takes a significant chunk of CPU capability away. On my system, the Boot Camp installation scored 308 MB/s for the CPUCompression2Metric and 470.9 MB/s for the Encryption2Metric, versus 152.5 and 223.0 for the same metric under Parallels. For those two tasks, you’re essentially losing half of the CPU by running in a VM. The difference is even more striking in the two MacBook Airs, where the different CPU models account for part of the gap but the VM adds a further penalty.
Likewise, graphics performance in a VM suffers because Windows is unable to use the native Nvidia or Intel drivers and instead has to pass everything through virtualized graphics adapters. Both VMware and Parallels have decent drivers capable of delivering Aero support with transparency and other effects. All of those effects are smooth when running under Boot Camp, but I can see tearing and jerky movements in a virtual machine. The lower scores reflect the differences accurately
Surprisingly, one area of Windows performance actually improves dramatically in a virtual machine. Look at the difference in performance on the Mac Mini, where the WEI score goes from 5.9 to 6.9. The Random Read score is 1.2 MB/s under Boot Camp but increases to 2.7 MB/s when using Parallels. That’s a huge improvement.
On the two MacBook Airs, you can really see the hit that the Intel graphics take when they’re forced to run using virtual graphics drivers. The penalty is even worse because the VM only has 1 GB of RAM available, whereas the Boot Camp installation has 4 GB to work with. And once again you can see the effects of storage drivers. Under Boot Camp, the 128 GB SSD delivers Random Read throughput of 49.5 MB/s. In a VM, the same score is 182.9 MB/s, a fourfold increase.
In Boot Camp, the SSD in that MacBook Air performs far worse than an SSD should. By way of contrast, a Samsung SSD in a 2009-vintage Dell notebook earned 130.2 MB/s on that score. The SATA III SSD in the Dell desktop I’m using to write this post scores 209.2 MB/s.
The moral? No matter which way you run Windows on a Mac, you’re going to give something up If you use Boot Camp, Windows will probably get as much as it can from the CPU and graphics adapter, but you’ll pay a performance penalty in terms of hard disk speed. By contrast, virtualizing Windows unlocks the full disk speed, especially with SSDs, but you pay a penalty in CPU and graphics muscle.