What is Salesforce Lightning?
More than just a new, more modern-looking skin, it’s a completely different UI for Salesforce.com (SFDC), it really is mobile ready (if not yet mobile optimized) and it really has completely different technical underpinnings than SFDC’s Classic UI and VisualForce. SFDC’s engineers have put a ton of thought and hundreds of man-years of development effort into the user experience, technical infrastructure, APIs, tools and documentation (both the “Trail Heads” and classic reference docs).
Lightning has gotten significantly faster over the last couple of months, and users updating to Firefox’s Quantum release will find everything very snappy indeed. Chrome users now have a reasonable choice if they want to switch browsers.