How to find out if the database is slowing the site down?

This host already has a bad reputation but I have to prove to my client that it is the server only that is causing the slow speed of the site.

Note: When I upload an html file on to the site and browse it then it opens quick. It is only the wordpress site that is running very slow. I just copied the whole wp site on another server and ran it and it opened very fast.
So we can say there is no issue with the wp theme and since the html file loads fast on the sever we can say the server also is quite fine.
Could it be because of host database then perhaps?

When the host was told about the speed of the site they said:
“following cacheable resources have a short freshness lifetime. Specify an expiration at least one week. kindly minimize request size.”

PageSpeed gives a score of 25/100:
-Reduce server response time
In our test, your server responded in 20.9 seconds. There are many factors that can slow down your server response time. Please read our recommendations to learn how you can monitor and measure where your server is spending the most time.
-Optimize images
Show how to fix
-Leverage browser caching
Show how to fix

What to do next?

Does your host have phpmyadmin? If so, you can go to Status > Processes as shown below.

Spam the refresh button while your WordPress is loading to see what’s going on in the database in realtime. Maybe something like this will also be helpful in some way:

Hopefully someone else can chime in and provide more options. Personally I would simply set up a quick DigitalOcean server ($0.007 per hour used), install ServerPilot (free), upload the WordPress site, and then show the client that to prove that it’s truly the host.

install the query monitor plugin, it will tell you what Database actions are slowing your site down, you can then see if you can optimise your queries.

Caching will help alleviate a lot of the DB slowdown but a 20 second load time on the actual server is really bad, ideally should be under 1 second…