Hi,

I checked on your site .

It seems that you use Redirection on your website and i saw redirection_404 on your database is 1.8 G ( http://prntscr.com/nmg4i0 )

Please contact with author of plugin and as theme why  table on  database is big size. We think they will find what the issue.

then remove it and remove data come from Redirection  function on your site.

Then check this guide to know how to optimize speed on your site.

Best Regards,

PenciDesogn