Well, it's just one possible way to handle the problem you've created, there may be others.

I created. Of cource.
Thank for your "help".

If not then I would bookmark those pages or get them from history.

40 tabs to bookmarks? I have about 30 folders in bookmarks, and about 200-300 links.

Store tabs to bookmarks, and then open them back - Usability => 0.

Also extended lazy load is working correctly: focused tab loads and then other start loading progressively.

Set "ext lazy" to disabled. Then just 1 tab is active after start. PC and opera responsive after 3-7 sec., like opera not started.

Maybe upgrade the computer?
Sorry, there don't seem to be any solution other than deal with it.

leocg, 8Gb of RAM on 2133MHz, SSD, 2 cores 4 threads 3.5GHz - not enough for Opera????
16Gb of RAM and 8 cores 16 threads 3.5GHz would be enough?)) For 20-40 tabs.

nanopi, Installed opera 45.0.2552.898 and removed opera_autoupdate.exe - works great.

could you suggest other some browser have this similar feature?

daoluong1991, Vivaldi?

Maybe upgrade the computer?

16 cores + 32Gb RAM? Or android pad?))

Don't keep tabs opened.

Not a variant.
Other ideas?

is it the same as the Old Opera? Copy some files on appdata etc?

Old opera could save session. It was very cool. It already released in vivaldy.
But, newOpera - can't. newOpera - disappointment.

C:\Users<username>\AppData\Roaming\Opera\ and C:\Users<username>\AppData\Local\Opera - just backup it.
It even could save authorisation of opera sync))

Forum cuts _... Y_R

Opera 46
1drv.ms/i/s!AlW2wfAwY_RLiewL_rzy7_xtS5qZNg

Opera 46. Missing "Extended lazy session loading" at opera://flags.
opera://flags/?search=lazy

Opera 46 using too much RAM after start. It loads 10-15 tabs to memory. (I have 20 to 40 tabs)
It starts too slow, it lags itself, and lags all PC.

Returned Opera 45 - that's nice. Starts about 3-7 sec from HDD. Background tabs loads when i need.
Opera 45 = fast browser.
Opera 46 = slow, Very slow browser.

Opera 45
https://1drv.ms/i/s!AlW2wfAwY_RLiewMtY3cBFpWuRhu0g

Opera 46
https://1drv.ms/i/s!AlW2wfAwY_RLiewL_rzy7_xtS5qZNg

Windows 10 pro x64, Windows 10 pro x86, Windows 8.1 pro x86. All the same.
AMD A8-7600, 4+4Gb RAM 2133MHz, SSD Netac N5M 120Gb.

How to make Opera 46 not to load all tabs to memory by default?