This is more related to YouTube than anything else. When you disable lazy loading, the video iframe gets embeded directly which has direct access to YouTube’s API each time it get loaded.
For lazy loading, we fetch the thumbnail from img.youtube.com/vi/id/maxresdefault.jpg instead. There are multiple image sizes, but YouTube is not very clear on how they update these URLs. We use maxresdefault, which is the largest image they provide and, from my testing, is the one that usually takes the longest to update. In my case I found that thumbnails took anywhere from 1 to 30 minutes to get updated.
And to answer your last question, no, there is no way to set a refresh period. Thumbnail updates are not very frequent, so we don’t have a job that periodically updates them.