SAN FRANCISCO—Google Inc. on Wednesday introduced a change to its widely used Web search engine intended to speed up the time it takes to find and deliver results as it continues to fend off antitrust regulators' inquiries about the service.

The feature shows search results that change as each letter in a word is typed into the search box. Previously, a search began when a word was typed and the "enter" key was struck. The feature, which requires a user to be enrolled and logged into a Google account, appears in the U.S. and...