First, can you please help us by providing more information?  What version of DNN are you running?  If it's an older version, are you able to upgrade first?  There are always updates to various features, including those that help improve performance.  

For example, we're seeing some performance improvements just between three subsequent releases, from 09.10.02 to 09.11.00 to 09.11.01.  Each got a bit faster from what we can tell, and that's just the most recent releases. 

These days, the default performance settings of DNN can be mostly good,  Beyond that, you may be in the need of various maintenance tasks...  

1. Uninstall any extensions that are no longer being used (backup first!).  
2. Turn off any Scheduled Jobs in DNN you don't need.  
3. Empty your Recycle Bin in DNN.  
4. Put your DNN website behind a CDN and WAF.  CloudFlare starts as free and there is not need to use their paid plans for many folks.  This will also protect you from attackers.
5. If the SQL Server is on the same server as your DNN website, try to split them apart to distribute the load.  
6. Implement a maintenance plan in SQL Server to reindex at least once a month (all depending on your traffic and other factors).  Other maintenance tasks might be necessary.  
7. Figure out how large your database is, pull a backup, and then delete the data you don't need anymore.  For example, you might be able to delete all of the data older than 3-6 months from the EventLog, SiteLog, and ScheduleHistory tables.  Then, shrink the database and files. 
8. Begin checking your IIS (and other) logs to ensure the traffic you're serving is indeed traffic that should be served.  For example, do you need to block any bots or other automated scanners/tools so you're only serving real humans?  
9. Perform an integrity check on your website.  Ensure that there are no viruses or toolkits, and use the Security Analyzer to be sure nothing else DNN is aware of is wrong.  
10. Increase the server resources of your web/database server.  
11. Upgrade the OS/version of SQL Server Windows.  
12. Upgrade your server/hosting plan.  

That's just a general brain dump... There's more that could be looked into.  

Hi Will,
Thanks for your replay. We are using version 9.9.1 and in two weeks we will upgrade to version 9.11.1 in the test environment. I have already done most of the tinghs in your list. As i said, we doesen't get the problem with the regular users, but with the super user the site become very slow and sometimes we doesen't get a response. There's no log for this problem in iss / dnn log or db.

Superusers will always have a slightly slower page load.  Ideally, it should be a negligible difference, but it depends on the website and how it's put together.  Some modules do a TON more for Superusers than others.  So, be aware of that as well.  

Oh, and not everyone needs to be an Admin or Superuser. Ideally, both should be highly limited in the number of use accounts, and use role-based permissions instead. :) This will help a lot.

Hi Will,
Thanks for your replay. There is a way to give to a normal user the access to the admin bar?
As you suggest i would like to take out the super and admin role to certain user to speed up the login, but they need access to the configuration bar to handle the configuration for the site pages. Is there a way to do that? Like give to a custom role the access to the persona bar with only content tab displayed?
Thanks again

You can give a Role edit rights on a page.
Users in that role will then see a (limited) Persona bar to manage the page.
But there is not one central setting for that, it's per page