When it comes to running alternative operating systems such as Windows or Linux on the Mac there are several options. The first is to use Apple's Boot Camp, which runs Windows natively on Apple's hardware, without OS X running at the same time. Because of the dedication of all the systems resources to the OS this is the fastest and most compatible method of running Windows; however, it does come at the cost of not being able to access your Mac applications at the same time.
The second option is to run a virtualization program, which takes advantage of the virtualization technology in Intel's chips that allows a second "guest" operating system to run in parallel with the host operating system. This solution has become popular because of the convenience of having both OS X and Windows or Linux running simultaneously on the same system.
There are three major virtualization options available for OS X: VirtualBox, VMware Fusion, and Parallels Desktop. VirtualBox is a free option that is offered by Oracle, and will get you by if you need it, but it is slower than the other options and has a few glitches here and there. The real race is between the other two virtualization options.
Recently Parallels Desktop was updated to version 7, which along with a number of speed improvements has offered full integration with OS X Lion's new features such as full-screen applications.
For a while Parallels has been the front-runner when it came to virtual-machine performance. Still, VMware Fusion has remained a popular option, and today the company announced the availability of Fusion 4, which like the Parallels 7 upgrade brings speed improvements as well as integration with Lion's features.
Both companies are working to greatly optimize 3D graphics, implement 64-bit support in all aspects of their programs, and offer options to quickly get Windows up and running and integrated into your Mac work flow both in terms of how the Windows applications are presented and handled, and also how your files are shared between your Mac and your virtual machine.
If you use either VMware Fusion or Parallels Desktop, then upgrading to the latest version should definitely bring improvements. While Parallels has been ahead in performance comparisons between it and VMware Fusion, the latest version of VMware is claimed to be up to 2.5 times faster than its predecessor, so time and benchmarking will show to what extent Parallels' performance lead has diminished.
Parallels Desktop 7 is available as a $49.99 upgrade from previous versions, or for $79.99 as a full upgrade. VMware Fusion 4 is currently available for $49.99 for a limited time. Recently Parallels announced a VMware switch program that offers Parallels Desktop for $29.99 for those who switch from Fusion; however, with the release of the Fusion update some of the claims that Parallels makes may need to be revised.
Overall, if you are using a prior version of either of these virtualization programs, then upgrading to the latest version you should see a noticeable speed and compatibility improvement.