I use NuGet Package Explorer to open an existing packages a lot. When I enter [File] / [Open from Feed], the "Select packages" dialog opened.
I have few suggestions for improving usability and performance issue:
1. The default focus is on the search textbox which is right, but the focus will be lost after the feed been loaded.
2. When I press some keyword and press Enter to start search, I will choose one package to open. If I want to choose another packages with the same search result, I have to open the "Select packages" again and
re-type keyword and search again. I suggestion you may cache the last search result in this dialog for improving performance issue.
3. I think the results shown in the list could cache for just a few minutes for improving performance and provide a "Refresh" button for instant refresh the search results.