1) Install cache plugin.
2) If possible, decrease size of your images.
3) Deactivate needless plugins.
4) Decrease number of items (portfolios, albums, catalogs, photos) per page:
5)If you are using Wordfence Security plugin, deactivate Live Traffic, it extremely slows down sites.
6) Install a image compressing plugin, like https://wordpress.org/plugins/tiny-compress-images/ you can firstly try to check how they are compressed at tinypng.com
7) Check your server performance with http://www.bytecheck.com/ Normal time is 100 – 400 ms.
8) Go to The7 -> MyThe7 -> Settings -> The7 Post Types and Elements and disable all custom post types you don’t use. If you don’t use custom post types at all, please deactivate The7 Elements plugin – it will significantly improve page loading.
9) If possible, upgrade your PHP. The most recent version of The7 is compatible with PHP 7.
10) If back-end works slow, please address try to disable elements you don’t use.