They said they totally re-wrote all of the major parts of the engine. I think the performance and visuals were just a symptom of time, and starting from absolute scratch. Respawn started as an office with no furniture or workstations. It was a bunch of people in an empty room spitballing game ideas. They had to build an infrastructure to work in, rapidly scale up a new renderer, game engine and workflow/toolset with Source as a starting point. Titanfall took about four years, but I wouldn't underestimate how hard it would be to get Titanfall shipped. A lot of games take three to four years to develop inside companies that are already mature.