Lazy session loading: when restoring browser state from session, only the active tab will be loaded. Remaining tabs will be loaded when activated. Enable here: opera://flags/#lazy-session-loading
Extended lazy session loading: changes mode of operation of lazy session loading so that all tabs are gradually loaded in the background. Enable here: opera://flags/#extended-lazy-session-loading
I use my browser tabs as a to-do/to-read list – bad habit, I know. At any given time I have at least 40 tabs open. For people like me, features such as the above boost browser startup time significantly and help consume less memory.