title designer background video OFF
This is a performance issue, caused by the title designer. When title designer is open, selecting background video to ON obviously slows performance. So many people turn it off when editing. However, when you switch timelines, the background video automatically turns back ON, slowing performance again. The default state for the background video should be OFF. This results in the editor having to turn the background video OFF every time a different timeline is opened which is inconvenient especially for complicated projects with a lot of nesting.
Kevin Monahan commented
Recommend that you move off the Legacy Titler and move into the more modern Title Tools. Much more flexible when designing with backgrounds.