Move site from Wordpress multisite to singlesite + optimize speed/mysql + delete unused images

I want to move 1 site from my multisite (this site is not main site) wordpress to single wordpress installation (new clean installation). - the multi site is not version 4 (i think it is one before) but i want the newWordPress single site installation be 4.x 1. My site has around 400,000 posts and its phpadmin DB is around 1-2GB. 2. We cannot export/import using wordpress option because of the big size. Best solution: Because both sites on same server the hosting company can move the images folder + the DB tables (they online 24/7 so it is not a problem). - Than we need to replace the links to fit the single WordPress installation (plugin cannot do it because DB too big). This job also include: 1. Delete unused mages: I have a lot of posts i deleted but the photo stay so they take a lof of space, we need to find a way to remove them (maybe check if the image in some post and if not remove it + all its sizes). 2. Fix site speed (make it load pages fast) + fix MySQL error/too much process power: The site is very slow (yes i try the cache plugins and even CDN), there problem seems mostly because the mysql uses too much CPU process (i have dedicated server Xeon E3-1240 4x3.33GHz, that should not have a problem), do not know how, but i want to fix it - Server hosting company told me the problem with mysqL Plugins: 3. After we finish, i will try to add plugins (regular and once i need for auto posts) one by one and we see where we have time load problem, and try to fix it - This is auto blog that posts around 1000 posts per day.