I had a look at the Oxygen page, but they don’t provide any details of how they use VC. I suspect that means that they have fully integrated it, which means that, while you should be able to turn it off so that it doesn’t appear as an option for entering content, it will still actually be on. That means, unfortunately, that you won’t see any speed gain.
(By the way, if you need to buy a theme again, take a look first at the Mustang Lite theme here at wordpress.org. That’s written by a developer who sells an upgraded version that works with WooCommerce, and he uses all the bells and whistles that developer on ThemeForest do. But he puts them in a separate plugin, so you can turn them on and off properly, which is how it should be designed.)
In your case, you have a choice to make. Either just stick with what you have now, or else you have a long job ahead (but your menu links will work) because VC doesn’t provide any means of exporting content!
If you want the long job, first you need a plain text editor on your computer, like Geany, Pluma, or Notepad ++. Don’t use a word processor!
Then you need to open up Visual Composer on your first post, select all, and cut and paste it into your text editor (and save it with a helpful name, which will give you a backup). Now save in VC, so that it saves as empty.
Now paste the content of your text editor into the “real” WordPress editing screen. You might find that visual works better than html, or the other way round. Try it out and see. Don’t forget to save!
I’m afraid you’ll have to do this with every post and page you have. Not only that, but you might well find that you now have a lot of shortcodes that don’t work. Again, you just have to try it and see.
If you need something that you had before in VC but don’t have after doing this, you will need to look for a plugin to do it.
Once you have cut and pasted everything out of VC, you can find the option in your theme to turn it off.
