Thanks for contacting us.
Before getting to your question I'd like to clarify a bit how RadAutoCompleteBox works. When you provide an items source it gets initially sorted. This is needed since when the filtering starts, it is performed incrementally (each filtering step is performed on the result set from the previous) - which acts as a performance improvement. So, when you provide an items source containing more items, it may pose a performance hit when the control initializes.
I believe in your scenario the best approach would be to use a new feature that will be soon released which gives the possibility to use a third-party data source to provide filtered suggestions (like a webservice or database).
However, it will be great if you can send me a demo project that sheds some more light on your scenario so that I can directly take a look and see how I can help.
Please note that you will have to open a new support ticket in order to be able to attach your files.
Thanks for your time.
All the best,
the Telerik team
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