When Apple announced the iPhone 6s, they didn't mention that the new iPhones carry 2GB of RAM, an increase from 1GB on the iPhone 6. The 2GB RAM was later confirmed in Xcode and benchmarks. This increased RAM allows the iPhone to keep more Apps and data active in memory.
iDownloadBlog recorded this video showing the difference between the iPhone 6 and iPhone 6s after loading several websites in Safari:
The iPhone 6s is able to keep more websites active in memory without requiring a reload when returning to the tab. The additional RAM should also allow more apps to remain active in memory without relaunching.