That is not be quite right Ivan. Because indexing takes longer when there is more data in a file and because the issue compounds exponentially with a lower file delay setting this fix works. I've tested just now with a setting of 3600s for mp4's rendering from Vegas 17.
With the default settings I also noticed a big drop in performance when rendering video to my sync directory. After approximately 30 minutes of rendering an 8gb file (which should be finished in about 90 minutes) my render time crept up past 3 hours. I also noticed that my processor usage had dropped from 80% CPU / 13% GPU down to around 60% CPU / 6% GPU. I should have kept my eye on the Disk %.
My control is rendering to an un-synched directory. This took 1 hour and 28 minutes.
Rendering the same project to a synched directory with 3600s delay for *.mp4 took 1 hour and 32 minutes.