How to Make Chrome Not Load Tabs Until They are Selected?
At the time of this writing, Chrome does not support this functionality natively. I've been using The Great Suspender, an extension that adds similar functionality to Chrome.
You can verify the memory and CPU savings you gain while using this extension by opening the Chrome Task Manager: Menu 
> More tools > Task manager
Since Chrome 54 (Oct 2016) this feature is supported in Chrome natively, so previous bloated extensions are not that needed. A GNU GPLv3 licensed extension Native Lazy Tabs extension works and it is very small: 33 KiB.
Citing extension description:
Automatically discards all tabs at startup, before they load, reducing memory footprint and unnecessary bandwidth usage.
Native Lazy Tabs is a light extension for users that don't want to load all tabs when Chrome is started. It's native because it uses the native tab discarding method from the Google API (introduced in version 54). Also, because of that, it doesn't have the shortcomings of older implementations that could cause bugs like the pages reloading itself even after suspended.
Moreover, this extension doesn't use any memory because there's no need for a background page: it loads and unloads itself after Chrome is started.
Without any extensions it's possible to prevent loading tabs by disconnecting the internet when you start Chrome. Chrome will eventually give up trying to load the pages, and one-by-one tabs will gray out. Reduces CPU and Memory.
After reconnecting to the internet, a page will load when you select the grayed-out tab.
StefanQ points out you may also need to change these chrome://flags
- Disable Offline Auto-Reload Mode
- Enable Only Auto-Reload Visible Tabs
