Make old topic static but still searchable

[RedFoxy]

After I deleted about 1,5 milions of posts, my database is 6Gb big, I'm tring to find a way to make whole board light and less resouce eater, I had changed the web server from Apache 2 to Nginx with cache+php-fpm, I use sphinx search engine (an old vbulletin.com post), moved pictures from DB to physical files, but the board becomes always slower, now it runs on a dual Intel Xeon E5506 at 2.13GHz with 16Gb of ram, my knwoledge are limited, I'm tring to make faster webserver and mysql server (v5.6.35), now for $$$ question I must change server with a one less powerfull, I got a new and cheaper server, it's a Intel Xeon X3440 Quad Core with 16Gb of ram, and I must fit my board here :P