Technically it is possible to virtualize RadMenuItems so they will load faster. But from UX point I'm not sure how usable it will be to have 100 subItems.
I've attached sample project with modified RadMenuItem templates that includes custom scroll and item virtualization. There are 5 top level items each with 100 sub items and each of them with 100 sub items (3 level deep menu). There is also RadContextMenu which is also virtualized. (this is Silverlight 4 project with 2010.Q2.SP1 trail assemblies).
I hope that this example will be helpful.
Let us know if you need more information.
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