Rendering video is one of the most CPU-intensive processes for your computer. Unless you’re running with the latest and best hardware, it’s likely that while your computer is rendering video (especially HD), all other programs become impossibly slow. If you have a PC, there’s a simple way to reduce the priority of the video rendering activity so that your CPU is freed up to perform other tasks (like check email, browse the web, etc). Of course, doing this will also slow down the video rendering, but perhaps that’s a minor consolation for being able to actually use your computer during the video rendering process.
Do this: open up the Task Manager. Hit the Processes tab and find your video rendering program. Right click on it, hit Set Priority, and either choose Below Normal or Low. And bam!