I am having trouble with the feedback module on a child portal. When I fill in all the fields and submit the question, it keeps on loading and as a consequence the parent portal also has performance problems.

Some background information; I have used the feedback module within this specific portal twice on different pages for visitors to ask questions about different topics. When I test the module, the question does not reach the e-mail address I entered in the feedback module settings but I do receive copies of my questions.
 
Does anyone have a hint?

Not sure, I would first of all double check DNN is correctly setup for sending email:
How to set up email in DotNetNuke

I would also look at the event logs and look for any errors related to the feedback module, this may provide some clues, thanks,
Site logs, Event logs and Scheduler

This weekend I was able to read and watch your proposed tutorials and test some things on our portal.

It seems that the SMTP settings were already correct, although it takes about 30 seconds to get the message that the test e-mail is sent correctly. Is that a hint that something is wrong?

Furthermore I found out that it is the option 'send a copy' which slows down the process. When I turn this off, the e-mail is send correctly to the address specified in the module settings at a reasonable speed. When I turn this option on and submit a question, the page keeps loading and the e-mail is just send to me.

I viewed the event logs but could not find any clue (only that something is wrong with the media and event module, the next problem I have to fix).