For those of you who have written fairly complex PyQt applications, what tips and tricks would you offer for speeding up your applications? I have a few examples of where my program begins to slow down as it grows larger:

I have a 'dashboard' written that is destroyed and re-created when a user clicks on an item in a TreeWidget. What would be a better way to have a modular interface where clicking an item in the TreeWidget changes the dashboard, but doesn't require destroying a widget and recreating it.
Each dashboard also loads an image from a network location. This creates some slowdown as one navigates around the application, but after it's loaded into memory, 'going back to that same dash' is faster. Is there a good method or way to run a thread on program load that maybe pre-loads the images into memory? If so, how do you implement that?
When you have a large variety of dashboard items and data that gets loaded into them, do you guys normally thread the data load and load it back in which each thread completes? Is this viable when somebody is browsing around quickly? Would implementing a kill-switch for the threads such that when a user changes dashboards, the threads die work? Or would the constant creation and killing of threads cause some sort of, well, meltdown.

Sorry for the huge barrage of questions, but they seemed similar enough to warrant bundling them together.
I'm not sure if this is exactly the same thing you are doing, but it sounds similar to something I have in some apps, where there is some list of custom widget. And it does significantly slow down when you are creating and destroying tons of widgets.
If its an issue of lesser amounts of total widgets, but just being created and deleted a lot, you can just create the widgets once, and only change the data of those widgets as information needs to be updated... as opposed to creating new widgets each time the information changes. That way you can even change the data from threads without having to worry about creating widgets.
Another situation is where you are displaying a list with custom widgets and there are a TON of results. I notice it always slows down when you have 1000's of custom widgets in a list. A trick my co-worker came up with was to have a fake kind of list where it is using a static number of slots in the display. Say, it shows 10 slots in the view. The scrollbar doesn't really scroll down across MORE widget...what it does is scrolls the DATA through the 10 visible widgets. You can a crazy performance increase doing that. But only if it is an acceptable display style for your application.
Optimizing your PyQt applications