NWF$("span:contains('Item added successfully')").replaceWith("");
However, when submitting items over a slower network, I'm able to see the "Item added successfully" text for couple of seconds.
Is there a way that I can hide it completely or at least make it almost not visible?
Try to inspect the HTML markup of that fragment and use the advanced CSS selectors. I don't have a sample available to help you with an exact solution. Something like element[attribute*=value] or another specific selector independent of site locale (if you're multilingual) might prove useful.