webserver in DMZ - web site not unreachable or very very slow

Hi,

as you may have guessed I am new to the fortigate world.

Now users have noticed that it has become very slow and difficult to reach the company's website which is hosted on an internal web server in DMZ.

With Firefox 50% of times, the website does not open. The other 50% of the time is very slow, it takes even 10 seconds to open.

With Chrome, it's very slow, 10 seconds.

Why? what can i check? where are the logs?

Tnx