Do you experience the same delay if you type multiple words and press Enter? If not, the reason probably is that with a single word Firefox first tries to lookup/check whether any domain by that name exists, and only when that fails does it redirect to the default search engine URL. However, with multiple words it goes to the search engine directly since spaces are unsafe in URLs and must be encoded.
One possible way to make searches faster for single words from the Location Bar might be to add a keyword via the Manage Search Engines dialog:
This would of course require you to type "g cat" instead of just "cat", which is why I prefer to simply press Ctrl+E or Ctrl+K and start typing in the Search Bar directly.
Edit: As requested, to turn off the Domain Guessing feature:
In the Location Bar, type
about:config and press Enter
If the "This might void your warranty!" warning page appears, click "I'll be careful, I promise!" to continue
In the Search field, type
browser.fixup.alternate.enabled to set its value to