Takayuki Miyoshi on "[Plugin: Contact Form 7] Contact Form very slow after click submit"

Tried con versions 4.3.3 and 4.5.1.

After clicking the SEND button, the spinning will spin for about 30 seconds or more until show that the message was successfully sent.

At first, I thought it was a server problem, contact with Administrator and they said me everything was ok.
Watching the logs for MTA (sendmail) shows that from receivent to send the max time is (5s)

Tried deactivate all plugins relevant doesn't work.
There are no javascript errors when showing the page or clicking the button.
I've read about Javascript conflict but I'm not sure it that can affect the site.

I can't switch the theme in production due to current visitors. Duplicated the entire WP in another hosting and the slow sending error still preserves. (Not same server config).

What can I try next?
Thanks.

https://wordpress.org/plugins/contact-form-7/