Hello Computer Surgeon,
Basically, Christopher Bishop is correct and since your setup requires so long to load, it is best to revise it and use the ASP.NET techniques to improve the performance, e.g use AJAX, etc.
In case you prefer to keep the things as they are, the only thing I can suggest is to set VisibleDuringInit="false" and while the page is blank to show a DIV element with a loading image in it - when the splitter is completely loaded - just to hide the DIV. In this manner, the user will have a visual indication that something is going on in the background and it will be much better and user friendly than having a blank page.
All the best,
the Telerik team