First, please note that the XtraGrid does not put any data into its internal buffer, so, the slow down the grid introduces to your application can only be caused by painting. You may learn more on how the grid works with data if you read the XtraGrid Data Binding Specifics article.
Please try to perform the following test. Unlink the XtraGrid from its data source and run the app. Do you see a big difference between your initial project and the newly created one? If yes, we suggest that you implement a data loading routine in the background thread. Please review the "Give Your .NET-based Application a Fast and Responsive UI with Multiple Threads" MSDN article available at the following URL for more information and some code samples which illustrate this in detail:
In addition, please take a look at the articles available on the following links. They describe how to work with threading correctly using a very simple sample:
Also, here are a couple of useful articles, which contain a sample project demonstrating how to populate a DataGrid with data from a background thread:
You may implement the same approach if you are using the XtraGrid in your application.
You may slightly improve the DataLoading process and completely eliminate the grid painting effect if you call the View's BeginUpdate method before and EndUpdate after your query is opened.