3) We have installed cache plugin, enabled optimization options and page speed is not bad as it was before http://prntscr.com/pq9x0d https://tools.pingdom.com/#5b87743b1ec00000
If you improve the page size (reduce it to 1-2Mb) then it will be faster. You can use https://tinypng.com/ service to optimize images as gtmetrix recommends http://prntscr.com/pq9zkp and reduce the page size, for example http://prntscr.com/pq9y6z
4) Replace all the images from our site by your own http://prntscr.com/pq9ylt
5) Contact your server provider and ask if they could improve any things like server response time etc.
Regards