Why does masking slow render time so much?
So the first comp I made had this problem, though I didn't realize it then. I had a comp I was making and i didn't want parts of it to show up so I added a black solid and multiplied it with the video, whitening the areas I wanted to show. The resulting comp took almost 3 days to render. The next comp I made I didn't need to do this and it rendered at an exponentially faster rate, only 3 hours, even thought it was longer and had more effects. Today I went to render, using a mask again, and left for work. When I came back the estimated time had risen to 44 hours and it was maybe rendering a frame every 2-3 minutes. I thought maybe the effects were slowing it down so I deleted all of them, planning to use another program to added them. I restarted and still had the massive lag in render time. Then I got the idea to delete the mask and boom, it's racing through the frames and finished in 4 hours.
Why does adding a mask slow things down so much and is there a way I can stop this from happening?
