There could be lots of problems.
What version of Windows and Vegas are you using?
Have you monitored any of your resources?
Is your hard drive nearly full (under 10% free space)?
How much RAM do you have?
Have you viewed your RAM usage (such as in task manager)?
What CPU do you have and is it being used at 100%?
Pagefile size and usage?
How much memory do you have allocated to the “dynamic ram preview”?
Are your source clips on the same drive and/or same partition as what you are rendering to?
Have you noticed anything else about your machine, such as slower start/shutdown times? Other applications not running as fast?
Hopefully these will help you in finding the issue.
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https://www.davelozinski.com/DemoReel/
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