I've been having this problem with Opera since some recent versions.
My system is:
Windows 8.1 64 bit
So let's strike out possible system's problem/requirement, since since we all know Opera SHOULD run fine on everybody's specs here.
Let's also agree that we would like Opera to start and open up tabs from previous sessions, I believe this is an important feature that none of us would want to compromise.
After a lot of tests, deleting/recovering, disabling/enabling and so on, I've come to the following 'preliminary' assumptions:
Symptoms: noticeable lags when opening and closing tabs, playing video, typing, scrolling
The session db seems to be unrelated to saved browsing data and history, since cleaning up Opera's browsing data (Ctrl+Shift+Del) doesn't help.
Even after deleting site data - cookies and so on - the problem still persists.
There seems to be no difference between pinned and unpinned open tabs
Disabling all extensions doesn't help, so it's not their fault that Opera lags.
Deleting session.db and session.dbak. I tried leaving the journal file and haven't seen and unwanted effect, so just let it be...
As mentioned, disabling extensions doesn't help when Opera is already lagging and session is already big, BUT disabling certain extensions, such as Autopagerize (all sites concerned) seems to slow down the expansion of this session file.
This leads me to believe:
Maybe there's a bug in Opera that doesn't simplify/decrease this session file, leading to expand in size and as a result, slows down Opera.
Maybe some extensions help to worsen this bug, adding more stress to the file.
It seems that as long as session.db maintains its size under some MB (preferablly under 1MB), Opera runs fine. So keep an eye on your file.
Please share your experience on this issue and also maybe solution too.