I am realizing that always I insert a text (even default text / legacy in V11) on the video and add an effect (e.g. "shadow"), the preview isn't good.
Considering just the text, without the effect, the preview is smooth, perfect. But with effect it delays, jump frames etc. Removing the effect and come back great!
I proceeded with test in Vegas V11 (32 and 64 bit) and in V10 as well. The same bad preview.
Is it the the Vegas behavior? Any workaround?