Simple site, but speed is too much low

I'm actually kind of curious what you're using to determine that your site is 'slow'. Checking with an artificial slowdown from Chrome's Developer tools seems to indicate no significant problems (yes, it was three seconds to interactive with a simulated slow mobile link, but that's not all that horrible honestly).

From a quick look though, things you could improve include:

• Better image compression. Realistically you probably have to keep serving JPEG or PNG images to support Safari and Internet Explorer, but for everything else I would encourage looking into WEBP, it should net you better compression, and thus faster image load times. You should be able to find WP plugins that can handle this for you.
• Switch to a font that doesn't need to be loaded over the network. Specifically, either use a generic family ('serif', 'sans', 'monospace'), or use a known web safe font (like Georgia, Arial, Times, Veranda, or Tahoma). The font you're using loads reasonably fast, but not anywhere near as fast as one that the system already has installed locally.
• Audit your WP plugins and figure out what you actually need and what's just extra. Also look into whether you can find alternatives to what you're using that are more efficient. The stuff you are using is loading a lot of unused CSS and a significant amount of JS code as well, and if you can find ways to reduce that, you can probably speed things up significantly.
• Look into lazy-loading your images, especially ones which are off-screen on initial load. You should be able to find a decent number of WP plugins that can do this for you, and it's one of the easiest ways to make an image-heavy site like yours load more efficiently. Smush seems to be one of the most popular options for this, and it can also cover the image compression stuff I mentioned above.