Actually the RadAutoCompleteBox works a bit differently. The control works upon callbacks, in order to request its underlying (associated) data source. In other words, once the end user types/deletes a character in the input of the control - the AutoCompleteBox triggers a callback, in order to acquire the result set, base on present text in the input.
There might be several reasons for the experienced issue:
1. Slow/problematic connection with your database.
2. Enormous amount of data, that needs to be searched for the corresponding results in the database.
3. If the underlying data is enormous - the returned results also will be a lot, which would need additional time to render the results on the page.
However, regarding the your question - you can use the OnDataSourceSelect serve-side event of the control, which is triggered, just before the result items are requested from the underlying DataSource. At the handler of the event, you can build a custom query, in order to obtain a preferable set of results. For example you can get the first 100 results, regarding the text of the input in the control. Please refer to the following documentation article, providing more information on the usage of the event:
On a side note, I would like to ask you to submit your inquiries for our product in the corresponding forums. In other words, please make sure that you have set the correct Product for the ticket/forum that you are about to submit.
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