With that kind of volume you may need to spend money on a decent MySQL query analyzer to figure out which queries from WP (and WP plugins, most likely) are eating up your CPU cycles. For a cheap and quick (but powerful) first look at your problem, install mytop to see what your mysql and threads are doing while your system is running.

I've installed mytop.

Thank you very much.

In mysql-slow.log only appears this query many times:

SELECT count(*) as c FROM wpposts WHERE ID IN (SELECT objectid FROM wptermrelationships WHERE termtaxonomyid = 542)

Any idea?

Why is your droplet so massive? you don't need something that big for 300 concurrent visitors. I suggest you to start looking for holes in your droplet security, maybe is something else other than WP, do you have any other sites or php scripts running?

Yous MySQL is running on its own droplet? or into the same droplet as your website?
Also can you let us know more about your droplet environment, webserver, php version, mysql version, etc?
How is Total Cache configured?

Hello,

My MySql is running into same droplet that my website

I dont have any other site or php scripts running.

Thank you for help

I think I found the problem, it's caused for plugin WPtouch Pro

https://wordpress.org/support/topic/slow-queries-possibly-slowing-down-the-site