Upgrading takes forever

[aviv_el]

I've started upgrading my forum (in a staging enviroment) from 3.8 to 5.0. I won't say it all went smoothly, but it worked reasonably well. Until, I got to the stage of upgrading to 5.0.0 alpha. There's the building of the node table. It's been way over a day, on a server pretty much dedicated to this only. My forum has 11,385,494 posts. My guess this is why it's taking so long.
As I don't want my forum held hostage for more then 3 days, just to get the upgrade done, I was wondering if there's a way to upgrade while the forum is still running. Now, I get this error, saying "There is a serious error and the page cannot be rendered" (a windows error message). Anything I can do about it?
Thanks in advance
Aviv