Virtual machines are great for running another operating system on top of your desktop, like testing out Windows 8 or running OS X inside Windows. They can be notoriously slow, though, and using a fixed size disk can help you speed things up.
Our friends over at the How-To Geek round up a bunch of tips for speeding up your virtual machines. Some you may already know, like allocating more RAM or CPU resources to the system, while others you might not. Here's one we didn't know about: using a fixed size disk can actually make your machine run faster than using the space-saving, dynamically allocated disk—particularly when you're writing large amounts of data to your virtual machine (big file transfers, downloads, and so on). It'll also decrease the amount of fragmentation your disk experiences. To use a fixed size disk, just choose the option when prompted during the initial setup of your machine.
The How-To Geek has all sorts of other performance-boosting tips in their guide, from placing VMs on another drive to excluding those files from antivirus apps. Hit the link below for the full guide.
The HTG Guide To Speeding Up Your Virtual Machines | How-To Geek