Do you experience the slow loading problem in Internet Explorer 7 and 8? Do you use <telerik:RadScriptManager> in the page with the RadSpell controls?
My suggestion is to create a simple aspx page with a standard <asp:ScriptManager> on it, put a couple of RadSpell controls and textboxes and test the loading speed in IE6 and IE8. Does the problem still persist?
the Telerik team