You return whatever results you can before the user perceives a delay. If you think a web search or slower search is useful (it isn't) then add a button to expand the search such as "Search the web for ${search term}" or "Continue this search in Explorer".

mulmen's laws of UX design:

1. It is never ok for the user to perceive input delay.

2. There must be no compromise in service of law #1.

3. Your UX improvement is a regression until proven otherwise.

4. To change the UX you must first destroy something you love.

5. The CPU belongs to the user, not to you.