This is a relatively common complaint. What it usually comes down to is the Default Dashboard that is displayed as your Home Page when you log in. While the dashboards are useful in getting a high-level overview of everything that's going on, these reports take a lot of time, particularly if you have a lot of incidents in the system. If you're using a default dash, it's probably got 6 reports that it's trying to compile for you every time you log in. I've seen these take upwards of 20 minutes in some cases to load.
I suggest that you define a default dashboard for yourself that only has one or two reports in it...or even better if you want to log in immediately and don't care about the dashboard, define one with no reports at all.
In my experience, UI performance issues are almost always just the loading of the dashboard. Other reports in the system and navigating between incidents don't tend to be a problem. So you may need to forgo the convenience of using a dashboard, but the benefit of being able to actually log in immediately when you want to should outweigh this.
If it's not just the dashboard, but navigating in general through the system, then it could be database related, and if you go through Support they'll advise you to run the DB Diagnostics to compile some statistics that might help them help you identify other issues that may be impacting your performance.
~Keith