That is unusual to see this problem in VEGAS Pro. It is a known problem with VEGAS Movie Studio Platinum, but I have never seen it happen in Vegas Pro.
The problem is most likely being caused by how Vegas is reading any third party OFX Video Effects you may have installed on your computer. The more 3rd party Video FX you have installed, the worse the problem gets. The specific folder that causes the problem is this one: C:\Program Files\Common Files\OFX\Plugins
If you temporarily move any plugins out of this folder and boot up Vegas, you should see no delay at all. However if you do this, these Video FX will not load in Vegas.
If you see many different FX in the OFX/Pluigins folder, I would recommend going through a process of elimination and take everything out of the folder (temporarily), then put one FX folder back in at a time and re-boot Vegas each time. Keep doing this until you find which FX are causing the problem.
If you can't get it working, take a screen shot from inside the OFX plugins folder and upload it to the forum here, so we can see what you have installed. C:\Program Files\Common Files\OFX\Plugins
I don't have that folder but I have tried removing VST plugins that I have installed and it doesn't seem to fix the issue. I will try resetting movie studio 13 since that was the one I also had on my computer.
EDIT: I fixed Movie Studio 13 but VP15 is still broken