This has been explained to you before, by me and others.
Once again, ANY video effects, envelopes, transitions, levels, pan/crop, etc., etc., etc. must be rendered on the fly in order to preview. Vegas does NOT automatically background render such as Avid.
That is why Vegas includes features like Preview Render, RAM Preview Render, and Render to New Track, in order to make your life easier. Hope you can wrap your brain around this one, rather than starting a new thread every time you run across the behavior (by my count a dozen new threads on the same topic over one month's time).
Here is my exact quote from YOUR PREVIOUS THREAD "Media/Text generators are the same as transitions and effects, in the context that in order to preview them in real time, Vegas must render on the fly.
Because restarting Vegas flushes all its memory and undo/redo buffers. I save and restart Vegas routinely after every couple of hours of editing, or when the preview stutter starts to get worse. And without fail before each render. Been a habit of mine for eight years, going back to Vegas 2.