I wonder how you got two testimonials menu in the wp-admin . It must be a plugin issue .
Pls deactivate unwanted plugins since it will result slowdown the site .
I have deactivated some thirdparty plugins and the result is very better .
Your site is not letting me to edit the contents . Pls see the screenshot .
Can you pls try now to add testimonials ? Hope it will work now .
BTW you can activate one plugin at a time and see which plugin is causing this issue .
But using a number of plugins will slowdown your site . Pls use the other plugins if necessary .