Rule 1:

Always add a custom thumbnail to videos. The vimeo UI makes the element look messy.

Rule 2:

Keep your files sizes as small as possible. Like 100-200kb JPGs (never PNGs) to maintain some quality, but go as low as you are comfortable with.

Your page load speed can negatively affect the user’s experience of your site, which is why Google will punish you with a reduced SEO.