Don't load tab's contents until user switches to it

[DelusionalOptimist]

Current behaviour
When restoring tab groups, there is a spike in memory consumption and webpages load slow. This probably happens because all of them are trying to load at the same time.

Desired bheaviour
Once restored, load a tab's contents only when the user switches to it.

I'm not sure if this is achievable or not but I'd like this to be considered once. Also, Firefox does this by default when restoring tabs on startup.