An option has been added to the options page: "Defer loading embedded content until it's visible". It is disabled by default.
When enabled, videos and other embedded content will be loaded as the users scroll to them. Pages with many videos will load faster and use less memory, but scrolling through many videos will feel less smooth.
If you use this feature, I'd like to hear your feedback. This is an experimental feature that may be removed depending on feedback, or lack thereof.