Fastest, huh? I'd say either Apple or Microsoft on the grounds that Apple controls the hardware and thus controls the experience, which in my experience is very responsive. Likewise, Microsoft has catered to relatively low end hardware while still maintaining a minimum standard to produce similar responsiveness. Google is completely open to any and all hardware, as well as open to third party software with little or not vetting, and the OS is natively powerful, which makes it difficult to maintain a consistent experience across all devices.
This is simply a logical view of the potential performance experience on each platform. Actual experiences are somewhat subjective and will vary. Though usually comparison videos will include a speed test for things like boot up, loading apps, and navigating to websites.