If performance is disastrous, put simply, users assume you're an idiot. They believe you do not understand there are problems, you don't know what the problems are, or you're not fixing the problems. Unless you have done something to change their minds, that's what they will think.

How could users be so cruel? Look at it from their perspective. They may believe it is solely your responsibility to keep the system running smoothly and that you get paid very well to do this. So if performance is unacceptable, it follows that you are not doing your job or are incapable of doing it well. That may be completely incorrect, but unless you do something quickly to break that thought process, it will become truth to them and, unfortunately, perhaps to others.

Breaking this destructive thought process is easier than you may think. If you have met with users about the key business processes and have timed those processes, you have already demonstrated you care and know what is important to them. If you have not met with them, then schedule a meeting right away. Time is of the essence.