I guess results may vary.
Despite the lack of an easy way to sync contacts, calendars and others with a smart phone which may be a show-stopper for some, Thunderbird is an overall excellent mail client. The funniest thing, is that my two main "selling" points with Thunderbird are exactly the speed at which the searches are executed and returned (which in my experience are so much faster in Thunderbird than in Outlook that I can't believe people Microsoft doesn't improve their software). The resulting behavior is that nobody typically uses the search in Outlook, they simply rather sort their inbox by sender, press the first letter of the sender and then scroll until they reach it. Not the most efficient way in my opinion.
The other killer feature for me in Thunderbird is a recent one, when a large file is attached to an email, a notice will show up at the bottom and will ask you to use the Filelink feature which will basically use UbuntuOne, Box.com and another service to automatically store and link files using one of these services. It is really a life saver for average non-geeks users which can seem to make a distinction between a 75k thumbnail JPG file and a 150 megs PDF catalog with pictures.
My typical user (used to) uses Outlook on a standalone computer using POP3 hosted on a third party company. No exchange. So if you want to compare apples and apples, I would say Thunderbird is faster at indexing, searching and display results than standalone Outlook. Maybe you are using Exchange in your example with the searches being indexed and returned from the server resulting in a faster execution?