The two main features for Opera 43 is a new feature called "Instant Page Loading" and enabling PGO, but the PGO build is only used by Opera on Windows.
Instant Page Loading is Opera's latest effort to speed up page load times after all their optimization work in earlier releases. Instant Page Loading is basically predictive loading depending upon the URL you are typing, similar to Google's predictive search behavior. Krystian Kolondra of Opera explained, "Instant page loading is a technology that predicts which website you’re typing the address for. Once recognised, it begins loading the site in the background even before you hit enter...Instant page loading becomes smarter over time by learning which URL inputs led to a specific website. For example, if you type “nyt.com” on multiple occasions, it will learn from this and begin loading the New York Times in the background. In addition, when you search for something in the address bar, it will load the results likely to be clicked in the background."
More details on Opera 43's release over at Opera.com.