For the fastest way to search for something on the web, simply speak the word Google first to Siri, followed by whatever you want to search for. This method is, by far, the fastest way to do a web search in iOS. And, it can improve your overall experience with Siri.
Here’s an example: if you ask Siri, “Ten best Hawaii beaches,” Siri will bizarrely respond with a weather forecast. Begin that same question with Google and the browser will quickly and automatically launch and point to several great articles listing the ten best beaches in Hawaii. By starting off with Google, I avoid having to repeat myself or navigating to the browser app.
Another reason to say “Google” first is that Google is also great at correcting mistranslated words or phrases and figuring out what you’re really looking for. Compare this to Siri, which often takes correctly translated words and delivers incorrect info. Don’t get me wrong, Siri is great for creating reminders, text messages, alarms, & timers, and a handful of other things, but Siri is not so great at answering most questions.
Also note you can say Bing or Yahoo to automatically launch searches in those engines as well.