@lando242 Windows 8.1 Enterprise with 8gb of Ram.
I deleted the session.db file and restore my tabs this morning and already the file is back up to 79mb. I'm not sure at what size it starts to get noticeable but its somewhere around 130mb or so, most noticeable when scrolling in web pages.

I'm on the verge of abandoning Opera for this reason.
Once the file gets large enough, Opera becomes pretty much unusable. I've been getting by using Session buddy and deleting session.db file and that work for a time but its not long before the file has grown large enough to become a problem again.