Its not about the track numbers its the number of videos that you have running really for the most part , because thats were the rendering is being slowed down its probably to much for your computer , The only way i can think of is to render Three , the first time render 2 out of 4 videos with the image in the background and then render the next 2 videos (Side note : place all the videos in the spots you want them for the finished product) in place then on a new project combine both if you understand how tracks work in vegas you can make them look like they've always been together with the final render . Im not sure if this will work for this case but this kind of method works for when you rendering something with alot of effects and just completly slows it down . This is basically a pre rendering technique .
